Add CHANGELOG entries for the observes extraction

[ci skip]

actionpack/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
+*   Sweepers was extracted from Action Controller as `rails-observers` gem.
+
+    *Rafael Mendonça França*
+
 *   Add option flag to `CacheHelper#cache` to manually bypass automatic template digests:
 
         <% cache project, skip_digest: true do %>

activemodel/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
+*   Observers was extracted from Active Model as `rails-observers` gem.
+
+    *Rafael Mendonça França*
+
 *   Specify type of singular association during serialization *Steve Klabnik*
 
 *   Fixed length validator to correctly handle nil values. Fixes #7180.

activerecord/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
+*   Observers was extracted from Active Record as `rails-observers` gem.
+
+    *Rafael Mendonça França*
+
 *   Ensure that associations take a symbol argument. *Steve Klabnik*
 
 *   Fix dirty attribute checks for `TimeZoneConversion` with nil and blank