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 guess just hope that most people will use a reset with Bitters. 
I'm thinking some thing like `Bitters is made to work with a CSS reset and not replace it. Our suggested reset is Normalize.'