Add CONTRIBUTING file

@BjarniRunar I noticed that you are copying and pasting a comment on the licencing situation to all the pull requests.

I added a  CONTRIBUTING.md file that will appear to users before they make pull requests. That way you have some automated way of informing contributors about the current licensing situation.

See here for more details: https://github.com/blog/1184-contributing-guidelines

Oh, and I'm fine with it being released under both the licenses ;)