Document Rails 4.x improvements to schema changes.
Move upgrade details into feature docs.

Duplicating the documentation in two places was causing some information
to be dropped. Additionally, the README.md file was starting to be
consumed by the upgrade notes.

- Use the feature documentation as the primary location for the upgrade
  docs.
- List the important topics users need to be aware of in the README and
  link them to the section of the upgrade guide
Move upgrade docs into a directory structure.

Extract out the upgrade notes from 1.x to 2.x so they are still easy to
find. Make the primary upgrade docs the main page.