Written by @iamnbutler 

This PR adds a basic CONTRIBUTING.md. It has a few links that need to be
added, which we marked as coming soon.

Here are a number of follow up tasks we need to do:

- [ ] Add CLA link
- [ ] Add public roadmap link
- [ ] Add link to channels doc once it is up
- [ ] Add link explaining how to find a channel related to your
contribution or or to create one

Release Notes:

- N/A