Update CONTRIBUTING.md to be more comprehensive

Comments welcome from ANYONE! This pull request updates our main CONTRIBUTING.md file to be more clear about what potential open source contributors to edx-platform need to do in order to contribute, as well as sets out expectations for both contributors and edX engineering.

@singingwolfboy @jtauber can you take a look here and tell me what you think?

@carsongee would you mind also taking a look and letting me know what might be missing or is unclear from your perspective?