Update actionpack/CHANGELOG.md

actionpack/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
+*   Templates without a handler extension now raises a deprecation warning but still
+    defaults to ERb. In future releases, it will simply return the template contents. *Steve Klabnik*
+
 *   Remove `:disable_with` in favor of `'data-disable-with'` option from `submit_tag`, `button_tag` and `button_to` helpers.
 
     *Carlos Galdino + Rafael Mendonça França*