Add CONTRIBUTING.md

Include guidelines for contributing to the project.

I think it will be worth adding a little more information to this, e.g., steps to set up your environment / general guidelines on workflow for this specific projects, not just how to use Git "properly". Hit me up with amendments and I'll update the patch.

Things to include:

- [x] IRC / Google Group details
- [x] How code should be submitted for review (by everyone). Use "ship it" to indicate you have properly reviewed a commit. Can't merge into master without at least 1 "ship it" from a repo collaborator.
- [x] Document tagging and publishing process for maintainers.

Fix #399