modify write_docs (how to contribute documentation) (#899)

* modify write_docs_cn, write_docs_en, include specifications about python 2.7.15 and creating  virtual env with conda

* gitignore update

* revert .gitignore;  update write_docs

doc/fluid/advanced_usage/development/write_docs_cn.md

@@ -56,8 +56,13 @@ sudo apt-get update && apt-get install -y python-dev build-essential
 ```
 git clone https://github.com/PaddlePaddle/PaddlePaddle.org.git
 cd PaddlePaddle.org/portal
-# To install in a virtual environment.
-# virtualenv venv; source venv/bin/activate
+```
+
+之后需要安装依赖库，请确保在python 2.7.15 或2.7.16 环境下安装。推荐使用Anaconda或virtualenv创建合适的虚拟环境后安装依赖库。
+
+安装依赖库：
+
+```
 pip install -r requirements.txt
 ```
 

doc/fluid/advanced_usage/development/write_docs_en.md

@@ -51,13 +51,19 @@ Take the ubuntu system as an example, run:
 sudo apt-get update && apt-get install -y python-dev build-essential
 ```
 
-then:
+Then:
 
 ```
 git clone https://github.com/PaddlePaddle/PaddlePaddle.org.git
 cd PaddlePaddle.org/portal
-# To install in a virtual environment.
-# virtualenv venv; source venv/bin/activate
+```
+
+Then install requirements. Please  make sure that you install with python 2.7.15 or 2.7.16. We recommend you to use Anaconda or virtualenv to create an appropriate virtual environment first. 
+
+Install requirements：
+
+
+```
 pip install -r requirements.txt
 ```