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)
What if I re–worded it to:

“Bitters should live in your project’s root Sass directory and we _encourage_ you to modify and extend it to meet your design and brand requirements.”