Create CONTRIBUTING.md

GitHub suggests putting a CONTRIBUTING.md in the repository. https://github.com/blog/1184-contributing-guidelines

Currently, we have such content at Confluence. We can either just link people to Confluence, move those confluence pages to this file, or forget about making this file in the repo.
I was referring to the github links to the teams. They don't seem to be accessible if you aren't part of an opensim github team.