Contribution Guidelines

I think it's great to have a bunch of community maintainers now and I'm excited to see some active development. Before redefining the world of mobile game develoment as we know it, I thought it would be a good idea to agree on some guidelines, so we're all on the same page as to how we want to work. :)

This is my proposition, in the form of a [CONTRIBUTING.md](https://github.com/blog/1184-contributing-guidelines) file. As always, there are exceptions to the rule, and nothing is set in stone. Let me know what you think, what I forgot, or if we need these kind of guidelines at all.