adds the release notes for Rails 4.0 to the guides index

This is still an early draft, but let's add it to the index
to have it ready.

guides/source/documents.yaml

@@ -145,6 +145,10 @@
       url: upgrading_ruby_on_rails.html
       work_in_progress: true
       description: This guide helps in upgrading applications to latest Ruby on Rails versions.
+    -
+      name: Ruby on Rails 4.0 Release Notes
+      url: 4_0_release_notes.html
+      description: Release notes for Rails 4.0.
     -
       name: Ruby on Rails 3.2 Release Notes
       url: 3_2_release_notes.html