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 
Sorry, I meant "off" there - I'm seeing a little crosseyed since I've been debugging a funky issue after working 12 hours today.

I don't consider it just for beginners, it's just more terse if you know what it's doing - which I prefer.