Update CONTRIBUTING.md (#332)

Author: luomai

CONTRIBUTING.md

@@ -1,13 +1,13 @@
 # Contributing to TensorLayer
 
-We appreciate contributions towards TensorLayer 
-either by adding / improving examples or extending / fixing the TensorLayer core library. 
-To make your contributions, you would need to follow the guideline:
-we are using [pep8](https://www.python.org/dev/peps/pep-0008/) coding style and [numpy doc style](https://github.com/numpy/numpy/blob/master/doc/HOWTO_DOCUMENT.rst.txt).
-The following tools are used to ensure that your PR comply the project convention:
+We appreciate contributions
+either by adding / improving examples or extending / fixing the core library. 
+To make your contributions, you would need to follow the [pep8](https://www.python.org/dev/peps/pep-0008/) coding style and [numpydoc](https://github.com/numpy/numpy/blob/master/doc/HOWTO_DOCUMENT.rst.txt) document style.
+We rely on Continuous Integration (CI) for checking push commits.
+The following tools are used to ensure that your commits can pass through the CI test:
 
-* yapf (format code)
-* isort (sort imports)
-* autoflake (remove unused imports)
+* [yapf](https://github.com/google/yapf) (format code), compulsory
+* [isort](https://github.com/timothycrosley/isort) (sort imports), optional
+* [autoflake](https://github.com/myint/autoflake) (remove unused imports), optional
 
-Please apply these tools in the changed files before submitting the PR.
+Please apply these tools in the changed files before submitting your PR.