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.
These links don't seem to be accessible to people that aren't in an Opensim team.

Also, shouldn't the text be wrapped?