Add CONTRIBUTING.md

[frreiss]

Create contribution guidelines for the project, and create additional issues as necessary to cover bringing existing code up to the standards described therein.

General outline:

• CONTRIBUTING.md file at root of project
• Pull requests encouraged
• Feel free to open issues with bugs/feature requests
• CLAs with IBM required for external PRs
• Copyright notices on new files
• Code style: TensorFlow standard, but with type hints. Code must run with Python 3.6 and parse with Python 2.7.
• Tests for new features strongly encouraged