Add contribution guidelines

[zakhenry]

This project is contributed to by many different developers often from languages outside of js, so it would be good to have a quick "this is how you add a language" tutorial. The testing harness is also non-obvious, so an explanation of how ava snapshotting is used would help

see also #54 where https://github.com/idkjs would have been able to more easily get reasonML included without testing issues