Option to overwrite project README when hydrating #588

"Oh no! It does this thing where it doesn't overwrite the readme, and we do a lot of badges and things in there. We should change that," @delucchi-cmu says, while hydrating desi-aap for the incubator.

I agree--if you're hydrating a fresh github repo, there's a good chance you clicked the option to create a blank README, so it'd be really nice to have it replaced with something more useful.