Create code of conduct and contribution guidelines

As this project is open source, we want to:

1. Foster an expectation of kind, welcoming behavior for all learners
   and contributors.
2. Help potential contributors know how to engage with the project
3. Know their rights to extend and use this code

I selected Coraline's Contributor Covenant as it's a pretty solid
baseline for code of conducts.

Tim Pope's "Note on Git Commit Messages" is also quite fantastic. Good
commit messages and commit bodies have saved my bacon many-a-time.