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 
Well, there's literally tons less characters, and I don't think automatic and magical are close to synonymous. How about this phrasing:

This feature was introduced to help give everyday users a convenient way to automatically configure directories having different behaviors based on a naming convention, while not surprising users with unexpected behavior by default. See [the directory structure documentation](https://www.relishapp.com/rspec/rspec-rails/v/3-0/docs/directory-structure) for more details.