Create CONTRIBUTING document

[jeradg]

We're getting a lot of community contributions to the addon, which is fantastic! It would be useful to have a CONTRIBUTING.md document with guidelines for contributors to the project.

This might include:

• What to do if you encounter a bug
• What to do if you have a feature idea
• A checklist of minimum requirements for a PR to be accepted:
  • Test coverage for new and changed features
  • Inline JSDoc comments (and a list of which tags are required and which ones are optional)
  • Adding the contributor's name to the AUTHORS file
  • etc.
• Standards of behaviour (civility, non-discrimination, etc.)

A good example from the ember.js core project: https://github.com/emberjs/ember.js/blob/master/CONTRIBUTING.md