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.
Order the changes by importance.

Put the more important changes at the top of the page so that they are
read first. Placing the helper file changes first as the other changes
reference `rails_helper`. The next most important change is the
disabling of inferred metadata; as it has been a common source of
upgrade questions.
A few more grammar changes based on feedback.

- Remove the reference to "magical" to clear up it's meaning
- Make statements more declarative by removing personal "you"
Merge pull request #1093 from rspec/rails-4-schema-changes-docs

Rails 4 schema changes docs