Fixed issue #53: added CONTRIBUTING.md

Diverted a little bit from the Roxy example. People will always be running slush against a named version, so no particular need for a dev branch, so I skipped that. Also, there is no dedicated mailing list for this project. I therefor opted to divert people to the Roxy mailing list, seemed reasonably appropriate, and a better option than the General Developer MarkLogic mailing list.