Update documentation to add cache explaination

This updates the documentation explaining how to setup any cache that implements the Doctrine interface. As the Symfony documentation don't clearly shows how to do this, I intend to add it here.

doc/providers/serializer.rst

@@ -71,3 +71,16 @@ The ``SerializerServiceProvider`` provider provides a ``serializer`` service::
         ));
     })->assert("_format", "xml|json")
       ->assert("id", "\d+");
+      
+Use a Cache
+-----
+
+To use a cache you can register it in the specified encoder needed. All you need to do is have a class implementing ``Doctrine\Common\Cache\Cache``::
+
+    $app->register(new Silex\Provider\SerializerServiceProvider());
+    $app['serializer.normalizers'] = function () use ($app) {
+    return [new \Symfony\Component\Serializer\Normalizer\CustomNormalizer(),
+            new \Symfony\Component\Serializer\Normalizer\GetSetMethodNormalizer(new ClassMetadataFactory(new  AnnotationLoader(new AnnotationReader()), $app['my_custom_cache']))
+    ];
+    };
+