First draft of a README

Just describing what's going on. I don't expect this framework to be in _huge_ demand, so I think a [bare-bones README](https://github.com/Carthage/Commandant/blob/readme/README.md) is okay for now.

Note that it's pretty heavy on boilerplate at the moment. I've filed #4 and #5 to extract more of the code from Carthage to solve that.