README restructure

* Make instructions more clear and succinct
* Better align with Bourbon README
* Use double prime marks in examples, per our guides
* Provide numbered instructions

In the future, I would like to tweak and bring clarity to the “Using Neat” section.

I’m also curious if the FAQ’s section would be better served as a Wiki page; thoughts?
Created: https://github.com/thoughtbot/neat/wiki/Command-Line-Interface