Review public-facing GitHub docs

[cassidyjames]

I'd like to go through and make sure READMEs focus on being great "advertisements" in the context of community project documentation:

1. Clear, concise description (what problem does it solve?)
2. Why it's useful (and who it's for)
3. How to contribute right now
4. Showcase (screenshots, examples, demos)

I don't intend to go deleting anything, but it might mean moving things around a bit to ensure the top/front of the README is nice and clear.