README + Misc. Refinement

General refinement to align with the README’s in the rest of the Bourbon suite:

- Add Refills logo
- Move “Submitting components or patterns” section into a `CONTRIBUTING.md` file
- Tweak installation instructions to be more clear
- Add Bourbon family links
- Update thoughtbot logo
I found that most other thoughtbot `gemspec` files use single quotes. In fact, I think we mostly use single except in Sass. This file had a mix of both (Line 4 is using single), so just wanted to unify.