Update our contributing file

[Xiaojieqiu]

As more people start to get involved in developing dynamo, we need to set up good practice on how to contributing to our project. The first thing we need to discuss is about updating our current contributing file so that users can follow. In addition as @dummyindex suggested, we can use pep8speaks (https://github.com/OrkoHunter/pep8speaks) to do PEP8 check on every PR and comments accordingly (non-invasive).