add list of extracted plugins to rails4 guide

guides/source/4_0_release_notes.md

@@ -69,6 +69,20 @@ $ ruby /path/to/rails/railties/bin/rails new myapp --dev
 Major Features
 --------------
 
+Moved to a Plugin
+-----------------
+
+With Rails 4 several pieces have been extracted. While Rails won't ship with these features anymore,
+you can simply add the extracted plugin to your `Gemfile` to bring the functionality back.
+
+* Hash-based & Dynamic finder methods ([Github](https://github.com/rails/activerecord-deprecated_finders))
+* Mass assignment protection in Active Record models ([Github](https://github.com/rails/protected_attributes), [Pull Request](https://github.com/rails/rails/pull/7251))
+* ActiveRecord::SessionStore ([Github](https://github.com/rails/activerecord-session_store), [Pull Request](https://github.com/rails/rails/pull/7436))
+* Active Record Observers ([Github](https://github.com/rails/rails-observers), [Commit](https://github.com/rails/rails/commit/39e85b3b90c58449164673909a6f1893cba290b2))
+* Active Resource ([Github](https://github.com/rails/activeresource), [Pull Request](https://github.com/rails/rails/pull/572), [Blog](http://yetimedia.tumblr.com/post/35233051627/activeresource-is-dead-long-live-activeresource))
+* Action Caching ([Github](https://github.com/rails/actionpack-action_caching), [Pull Request](https://github.com/rails/rails/pull/7833))
+* Page Caching ([Github](https://github.com/rails/actionpack-page_caching), [Pull Request](https://github.com/rails/rails/pull/7833))
+
 Documentation
 -------------