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