Improve serialization example

doc/index.rst

@@ -30,7 +30,8 @@ Usage
 For standalone projects usage of the provided builder is encouraged::
 
     $serializer = JMS\Serializer\SerializerBuilder::create()->build();
-    $serializer->serialize($data, 'json');
+    $jsonContent = $serializer->serialize($data, 'json');
+    echo $jsonContent; // or return it in a Response
 
 Documentation
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