add contribution guideline

[lgarithm]

Add CONTRIBUTING.md as suggested by https://github.com/tensorlayer/tensorlayer/community.
It should define coding style, doc style, and describe tools that we use to enforce them.

[lgarithm]

Currently we are using

• yapf (format code)
• isort (sort imports)
• autoflake (remove unused imports)

and we should also use

• pylint
• pydocstyle
  when most lints warnings are fixed

[luomai]

This is actually very necessary for us. Lets do it.