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.
Thanks for noticing that Jason. Maybe we can just describe what the teams are in the README.

`
Below, we refer to "Dev Team" and "Owners". These are Teams within the opensim-org GitHub organization.
`