README Updates

Making the `README` more consistent with `README`’s in the Bourbon family.

- Added syntax highlighting
- Added numbered list to instructions
- Swapped single prime marks in favor of double prime marks
- Added links to Bourbon family
- Tweaked the wording in “Credits” and added thoughtbot Twitter handle
- Renamed `LICENSE` to `LICENSE.md` to be consistent with `README.md` and [future `CONTRIBUTING.md`](https://github.com/thoughtbot/bourbon/pull/379)