[Feature Request] Documentation on Acceptance Criteria for Contributions

[GoodmanBen]

Current State: There is no documentation in the repo pertaining to how contributors should contribute to tfmigrate.

Desired State: As this repo gains a larger following, it will be helpful to have a readme for contributors that guides on acceptance criteria. Some thoughts as to what would might be helpful to see in a contributor document:

• Desired formatting for new issues and PRs.
• Whether PRs should be linked to issues to help prevent duplication of work.
• Proper way to run test suite (this led to some confusion for me personally while working on this PR).
• List of prioritized new features that the community can work on for upcoming desired releases.
• Known licensing issues to be aware of (if any exist)