Add pre-commit hook for contribute documentation.

* Also add a soft link for contribute.md to make github recognize
  this file.

CONTRIBUTING.md

+./doc/howto/contribute_to_paddle_en.md
\ No newline at end of file

doc/howto/contribute_to_paddle_en.md

@@ -47,6 +47,22 @@ Then you can start to develop by making a local developement branch
 git checkout -b MY_COOL_STUFF_BRANCH
 ```
 
+## Using `pre-commit` hook
+
+Paddle developers use [pre-commit](http://pre-commit.com/) tool to manage git
+pre-commit hooks. It can help us format source codes (cpp, python), check some
+basic thing before commit (only one EOL for each file, do not add a huge file
+in git). `pre-commit` tests is a part of unit tests in Travis-CI now, every
+PR doesn't fit hook can not be merged into Paddle.
+
+To use [pre-commit](http://pre-commit.com/), you should install it by
+`pip install pre-commit`, and currently, Paddle uses `clang-format` to format
+c/cpp sources. Please make sure clang-format 3.8+ installed.
+
+Then just run `pre-commit install` in your Paddle clone directory. When you
+commit your code, the pre-commit hook will check the local code if there is
+anything not suitable to commit, and so on.
+
 ## Commit
 
 Commit your changes by following command lines: