Update CONTRIBUTING.md with process and policy details

[dcmiddle]

CII Best Practices recommends having clear contributor guidelines including documenting the need for new test cases when new features are submitted. I recommend using this as a template: https://sawtooth.hyperledger.org/docs/core/releases/latest/community/contributing.html In particular add the new test requirement and the commit message guidelines. https://chris.beams.io/posts/git-commit/#seven-rules