Write up a CONTRIBUTING.MD

Would be nice to define a few things

Ideas
- Make note of code of conduct, etc boilerplate
- Setup some coordination and communication guidelines (eg, to make sure work is aligned/doesn't conflict with the project goals / timeline / current work)
- Setup Coding guidelines (naming, namespacing, style, formating, documentation, tabs, spell checking, linting, etc)
- Codify our review process
- Other things?

This will allow us to be both more consistent, more open to contributions, and also a place that we can catch up with changes done to procedures/expectations.

Random sample from the internet: https://gist.github.com/PurpleBooth/b24679402957c63ec426

Thoughts?