Add the Chinese and English "Contribute Code" documentations. (#220)

* Add the Chinese and English "Contribute Code" documentations.

* Provide some more detail messages in the documentations

docs/contribute_code_cn.md

+# 如何贡献代码
+
+我们真诚地感谢您的贡献，欢迎通过 GitHub 的 fork 和 pull request 流程来提交代码。
+
+## 教程
+VisualDL 目前使用[Git流分支模型](http://nvie.com/posts/a-successful-git-branching-model/)进行开发，测试，发行和维护，以下教程将指导您提交代码。
+
+
+1. Fork
+
+    跳转到[VisualDL](https://github.com/PaddlePaddle/VisualDL) GitHub首页，然后单击 [Fork](https://help.github.com/articles/fork-a-repo/) 按钮，生成自己目录下的仓库，比如 <https://github.com/USERNAME/VisualDL>。
+
+1. 克隆（Clone）
+
+    将远程仓库 clone 到本地：
+
+   ```bash
+   git clone https://github.com/your-github-account/VisualDL
+   cd VisualDL
+   ```
+1. 创建本地分支
+
+    所有的 feature 和 bug fix 的开发工作都应该在一个新的分支上完成，一般从 `develop` 分支上创建新分支。
+
+    使用 `git checkout -b` 创建并切换到新分支。
+
+   ```bash
+   git checkout -b my-cool-stuff
+   ```
+
+1. commit
+
+    在进行你的第一次之前，请先安装必要的工具，像是flake8, yapf 与 [pre-commit](http://pre-commit.com/) 工具。
+    VisualDL 开发人员使用 pre-commit, flake8, yapf 工具来管理 Git 预提交钩子。 它可以帮助我们格式化源代码（C++，Python），在提交（commit）前自动检查一些基本事宜。
+    不满足钩子的 PR 不能被提交到 VisualDL，首先安装并在当前目录运行它：
+
+    ```bash
+    pip install -r requirements.txt
+    pre-commit install
+    ```
+
+    VisualDL 使用 `clang-format` 来调整 C/C++ 源代码格式，请确保 [clang-format 3.8](http://releases.llvm.org/download.html) 版本在 3.8 以上。
+    请下载且安装在 path上. 例如 `/usr/local/bin`
+
+    Git 每次提交代码，都需要写提交说明，这可以让其他人知道这次提交做了哪些改变，这可以通过`git commit` 完成。
+
+    ```
+    git commit
+    yapf.................................................(no files to check)Skipped
+    Check for merge conflicts................................................Passed
+    Check for broken symlinks................................................Passed
+    Fix End of Files.........................................................Passed
+    Trim Trailing Whitespace.................................................Passed
+    Detect Private Key.......................................................Passed
+    Check for broken symlinks................................................Passed
+    Check for added large files..............................................Passed
+    clang-format.........................................(no files to check)Skipped
+    python-format-checker................................(no files to check)Skipped
+    [my-cool-stuff c703c041] add test file
+    1 file changed, 0 insertions(+), 0 deletions(-)
+    create mode 100644 233
+    ```
+
+1. 构建和测试
+
+    Users can build VisualDL natively on Linux and Mac OS X.
+
+1. 保持本地仓库最新
+
+    在准备发起 Pull Request 之前，需要同步原仓库（<https://github.com/PaddlePaddle/VisualDL>）最新的代码。
+
+    ```bash
+    git remote add upstream https://github.com/PaddlePaddle/VisualDL
+    git pull upstream develop
+    ```
+
+1. Push 到远程仓库
+
+    将本地的修改推送到 GitHub 上，也就是 https://github.com/USERNAME/VisualDL。
+
+    ```bash
+    # 推送到远程仓库 origin 的 my-cool-stuff 分支上
+    git push origin my-cool-stuff
+    ```
+
+1. 建立 Issue 并完成 Pull Request
+
+    建立一个 Issue 描述问题，并记录它的编号。
+
+    切换到所建分支，然后点击 `New pull request`。选择目标分支：
+
+    <img width="295" alt="screen shot 2017-04-26 at 9 09 28 pm" src="https://cloud.githubusercontent.com/assets/11692045/25436054/a6d98c66-2ac4-11e7-9cb1-18dd13150230.png">
+
+    在 PR 的描述说明中，填写 `fix #Issue编号` 可以在这个 PR 被 merge 后，自动关闭对应的 Issue，具体请见 <https://help.github.com/articles/closing-issues-via-commit-messages/>。
+
+    接下来等待 review，如果有需要修改的地方，参照上述步骤更新 origin 中的对应分支即可。
+
+1. 删除远程分支
+
+    在 PR 被 merge 进主仓库后，我们可以在 PR 的页面删除远程仓库的分支。
+
+    <img width="775" alt="screen shot 2017-04-26 at 9 18 24 pm" src="https://cloud.githubusercontent.com/assets/11692045/25436457/e4cdd472-2ac5-11e7-9272-badc76c4a23e.png">
+
+    也可以使用 `git push origin :分支名` 删除远程分支，如：
+
+    ```bash
+    git push origin :my-cool-stuff
+    ```
+
+1. 删除本地分支
+
+    最后，删除本地分支。
+
+   ```bash
+   # 切换到 develop 分支
+   git checkout develop
+   # 删除 my-cool-stuff 分支
+   git branch -d my-cool-stuff
+   ```
+
+至此，我们就完成了一次代码贡献的过程。
+
+## 提交代码的一些约定
+
+为了使评审人在评审代码时更好地专注于代码本身，请您每次提交代码时，遵守以下约定：
+1. 请保证Travis-CI 中单元测试能顺利通过。如果没过，说明提交的代码存在问题，评审人一般不做评审。
+2. 提交PUll Request前：
+   - 请注意commit的数量：
+     - 原因：如果仅仅修改一个文件但提交了十几个commit，每个commit只做了少量的修改，这会给评审人带来很大困扰。评审人需要逐一查看每个commit才能知道做了哪些修改，且不排除commit之间的修改存在相互覆盖的情况。
+     - 建议：每次提交时，保持尽量少的commit，可以通过`git commit --amend`补充上次的commit。对已经Push到远程仓库的多个commit，可以参考[squash commits after push](http://stackoverflow.com/questions/5667884/how-to-squash-commits-in-git-after-they-have-been-pushed)。
+   - 请注意每个commit的名称：应能反映当前commit的内容，不能太随意。
+3. 如果解决了某个Issue的问题，请在该PUll Request的**第一个**评论框中加上：`fix #issue_number`，这样当该PUll Request被合并后，会自动关闭对应的Issue。关键词包括：close, closes, closed, fix, fixes, fixed, resolve, resolves, resolved，请选择合适的词汇。详细可参考[Closing issues via commit messages](https://help.github.com/articles/closing-issues-via-commit-messages)。
+
+此外，在回复评审人意见时，请您遵守以下约定：
+1. 评审人的每个意见都必须回复（这是开源社区的基本礼貌，别人帮了忙，应该说谢谢）：
+   - 对评审意见同意且按其修改完的，给个简单的`Done`即可；
+   - 对评审意见不同意的，请给出您自己的反驳理由。
+2. 如果评审意见比较多：
+   - 请给出总体的修改情况。
+   - 请采用[start a review](https://help.github.com/articles/reviewing-proposed-changes-in-a-pull-request/)进行回复，而非直接回复的方式。原因是每个回复都会发送一封邮件，会造成邮件灾难。

docs/contribute_code_en.md

+# Contribute Code
+
+VisualDL is an open source project initiated by PaddlePaddle and ECharts.
+Our goal is to break through the deep learning black box and help the developers to see the training trend by providing visualization.
+We sincerely appreciate your contribution.  This document explains our workflow and work style.
+
+## Workflow
+
+VisualDL uses this [Git branching model](http://nvie.com/posts/a-successful-git-branching-model/).  The following steps guide usual contributions.
+
+1. Fork
+
+   To maintain the main repo and avoid dangling branches, please file Pull Requests from your fork. To make a fork, just head over to the GitHub page and click the ["Fork" button](https://help.github.com/articles/fork-a-repo/).
+
+1. Clone
+
+   To make a copy of your fork to your local computers, please run
+
+   ```bash
+   git clone https://github.com/your-github-account/VisualDL
+   cd VisualDL
+   ```
+
+1. Create the local feature branch
+
+   For daily works like adding a new feature or fixing a bug, please open your feature branch before coding:
+
+   ```bash
+   git checkout -b my-cool-stuff
+   ```
+
+1. Commit
+
+   Before issuing your first `git commit` command, please install [`pre-commit`](http://pre-commit.com/) and other requirements by running the following commands:
+
+   ```bash
+   pip install -r requirements.txt
+   pre-commit install
+   ```
+
+   Our pre-commit configuration requires [clang-format 3.8](http://releases.llvm.org/download.html) for auto-formating C/C++ code, yapf for auto-formating Python and flake8 for style checking Python.
+   Please make sure clang-format version 3.8 is available in your path. For example, under ```/usr/local/bin```
+
+   Once installed, `pre-commit` checks the style of code and documentation in every commit.  We will see something like the following when you run `git commit`:
+
+   ```
+   git commit
+   yapf.................................................(no files to check)Skipped
+   Check for merge conflicts................................................Passed
+   Check for broken symlinks................................................Passed
+   Fix End of Files.........................................................Passed
+   Trim Trailing Whitespace.................................................Passed
+   Detect Private Key.......................................................Passed
+   Check for broken symlinks................................................Passed
+   Check for added large files..............................................Passed
+   clang-format.........................................(no files to check)Skipped
+   python-format-checker................................(no files to check)Skipped
+   [my-cool-stuff c703c041] add test file
+    1 file changed, 0 insertions(+), 0 deletions(-)
+    create mode 100644 233
+   ```
+
+1. Build and test
+
+   Users can build VisualDL natively on Linux and Mac OS X.
+
+1. Keep pulling
+
+   An experienced Git user pulls from the official repo often -- daily or even hourly, so they notice conflicts with others work early, and it's easier to resolve smaller conflicts.
+
+   ```bash
+   git remote add upstream https://github.com/PaddlePaddle/VisualDL
+   git pull upstream develop
+   ```
+
+1. Push and file a pull request
+
+   You can "push" your local work into your forked repo:
+
+   ```bash
+   git push origin my-cool-stuff
+   ```
+
+   The push allows you to create a pull request, requesting owners of this [official repo](https://github.com/PaddlePaddle/VisualDL) to pull your change into the official one.
+
+   To create a pull request, please follow [these steps](https://help.github.com/articles/creating-a-pull-request/).
+
+   If your change is for fixing an issue, please write ["Fixes <issue-URL>"](https://help.github.com/articles/closing-issues-using-keywords/) in the description section of your pull request.  Github would close the issue when the owners merge your pull request.
+
+   Please remember to specify some reviewers for your pull request.  If you don't know who are the right ones, please follow Github's recommendation.
+
+
+1. Delete local and remote branches
+
+   To keep your local workspace and your fork clean, you might want to remove merged branches:
+
+   ```bash
+   git checkout develop
+   git branch -d my-cool-stuff
+   ```
+
+### Code Review
+
+-  Please feel free to ping your reviewers by sending them the URL of your pull request via IM or email.  Please do this after your pull request passes the CI.
+
+- Please answer reviewers' every comment.  If you are to follow the comment, please write "Done"; please give a reason otherwise.
+
+- If you don't want your reviewers to get overwhelmed by email notifications, you might reply their comments by [in a batch](https://help.github.com/articles/reviewing-proposed-changes-in-a-pull-request/).
+
+- Reduce the unnecessary commits.  Some developers commit often.  It is recommended to append a sequence of small changes into one commit by running `git commit --amend` instead of `git commit`.
+
+
+## Coding Standard
+
+### Code Style
+
+Our C/C++ code follows the [Google style guide](http://google.github.io/styleguide/cppguide.html).
+
+Our Python code follows the [PEP8 style guide](https://www.python.org/dev/peps/pep-0008/).
+
+
+Please install pre-commit, which automatically reformat the changes to C/C++ and Python code whenever we run `git commit`.  To check the whole codebase, we can run the command `pre-commit run -a`.
+
+### Unit Tests
+
+TBD