Readme doc updates

A variety of updates to the README. Two main goals with these updates:

- Be consistent in formatting
- Be consistent in using RSpec 3 syntax
I didn't remove it, just moved it to the top of the section: https://github.com/rspec/rspec-rails/pull/1117/files#diff-04c6e90faac2675aa89e2176d2eec7d8R173