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)
I like that but I'm afraid it will get lost in the paragraph. I'm not really sure of an answer to this and I like these changes overall and I don't want this to stop them.