Add a CONTRIBUTING.md file

I kind of agree with what @nimeshsubramanian  said before. This does seem to be mixing open source contributions and internal process. 

Looking at some other projects on here, contributing is pretty simple: https://github.com/cerner/cerner_kafka#contributing

There is then a separate document for repo maintainers to follow for merging, and committing.  https://github.com/cerner/cerner_kafka/blob/master/COMMITTERS.md

I don't think that example is the best one, but I think it does a decent job of separating out how to contribute to the project as a non-committer vs an owner/committer to the project. I'd prefer to see some of the parts here separated into a different doc similarly to how that one is split out.