Introducing a CONTRIBUTING guide

Note that I didn't mean to suggest the point *couldn't* make sense, I merely feel that the link as currently written does not reach the expected audience in a way that will draw them to the project. Especially consider that many simple changes would exist as a single commit, and those contributors are likely to be helpful even if they don't understand rebase operations. If a potential contributor gets the impression that the contribution policy is simply over their head, they are more likely to not contribute at all than to contribute something that might be imperfect but on the right track.