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 
In this case it definitely magical because it relied on a naming convention that wasn't always clear, mutating metadata from provided values and not necessarily matching up to an application structure (Rails itself has never enforced the folder structure).

I consider inferring spec type to be a legacy feature and believe the documentation should reflect that.