feature symfony#6707 Describe serialization config location in cookbook (jcrombez, WouterJ)

This PR was merged into the 2.7 branch.

Discussion
----------

Describe serialization config location in cookbook

Finishes symfony#6039

Original description:

 > Text from : http://symfony.com/blog/new-in-symfony-2-7-serializer-component-improvements

Commits
-------

80d0cdf Made the config location paragraph a bit shorter
8cffa78 Where to put config file to use the serialization group feature

Author: wouterj

cookbook/serializer.rst

@@ -171,6 +171,15 @@ to your class and choose which groups to use when serializing::
         $someObject,
         'json', array('groups' => array('group1'))
     );
+    
+In addition to the ``@Groups`` annotation, the Serializer component also
+supports Yaml or XML files. These files are automatically loaded when being
+stored in one of the following locations:
+
+ * The ``serialization.yml`` or ``serialization.xml`` file in
+   the ``Resources/config/`` directory of a bundle;
+ * All ``*.yml`` and ``*.xml`` files in the ``Resources/config/serialization/``
+   directory of a bundle.
 
 .. _cookbook-serializer-enabling-metadata-cache: