Rails 4 schema changes docs

- Restructure the docs to remove duplication
- Extract details from README to keep it focused and not be a full upgrade guide
- Add missing docs as noted in #936 
I sorta dislike the phrase "removing magic from RSpec", because I still think RSpec is pretty magical ;) How about something like:

This feature was introduced to help newer users by clarifying the existence of directories having different behaviors based on a naming convention, and also to give more-advanced users the ability to turn this off for more fine-grained control. See [the directory structure documentation](https://www.relishapp.com/rspec/rspec-rails/v/3-0/docs/directory-structure) for more details.