Community repo has a contribution guide

[BradleyBoutcher]

As with every project, the community repo is open to contributions from the external and internal community.

• [x] A CONTRIBUTING.md exists in the top-level directory
• [x] Details for internal and external contributors to improve documentation and clarity of community docs structure and content
• [x] Details for connecting a project back to the community repo through a README
• [x] Common conventions for making a project open for contributions