Added usage to documentation

Resources/doc/index.rst

@@ -116,6 +116,13 @@ De-/Serializing Objects
 The serializer supports JSON, and XML out-of-the-box, and can also handle
 many custom XML features (see below).
 
+The serializer can also be accessed via a Twig filter and will default to
+"json".
+
+::
+    {{ myObject | serialize | raw }}
+    {{ myObject | serialize('xml') | raw }}
+
 Versioning
 ~~~~~~~~~~