Add note about different behaviors of `recover` vs `restore`.

The `acts_as_paranoid` gem (v0.4.1) uses assignment followed by a call
to `#save` for restoring a record:

    # in #recover
    run_callbacks :recover do
      recover_dependent_associations(options[:recovery_window], options) if options[:recursive]

      self.paranoid_value = nil
      self.save
    end

This runs any `update` callbacks as well as `recover` callbacks.

Paranoia uses `#update_column` instead of `#save`, so no `update`
callbacks are run when restoring a record.

With this commit the difference in behavior is described in the section
"Acts As Paranoid Migration" of the README.