Make readme more clear and fix typos

- Cleaned up the ‘introduction’ to be a bit more clear.
- Made the instructions a proper ordered list
- Added a ‘Bourbon family’ section, as seen in the [Refills repo](https://github.com/thoughtbot/refills)
The biggest issue with Bitters communication is that people think / want it to sit in a vendor folder to use is as a glorified reset and not touch any of the styles. Can you better emphasis that it's not made for that?