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 
This whole block uses a lot of "you"s. For documentation I think it tends to sound better if it's a bit more impersonal. E.g.:

There are a few things to be aware of when using this:
  - Migrations still need to be run manually, although now they can be run in the 'development' environment
  - If the schema has not been initialized an exception will be raised instructing you to run `rake db:migrate` first.

If you wish to opt out of checking the pending migrations there is a configuration option made available in Rails' environment configuration. To disable this feature edit `config/environments/test.rb` to include:

```ruby
config.active_record.maintain_test_schema = false
```