update change log.

laravel/documentation/changes.md

@@ -33,6 +33,7 @@
 - Fixed bug in cookie retrieval when cookie is set on same request.
 - Fixed bug in SQL Server grammar for primary keys.
 - Fixed bug in Validator on PHP 5.4.
+- Added `format` method to message container.
 
 <a name="upgrade-3.2.1"></a>
 ## Upgrading From 3.2