include serializers in ActiveModel::Serialization example

activemodel/README.rdoc

@@ -168,7 +168,19 @@ behavior out of the box:
 
     s = SerialPerson.new
     s.serializable_hash   # => {"name"=>nil}
+
+    class SerialPerson
+      include ActiveModel::Serializers::JSON
+    end
+
+    s = SerialPerson.new
     s.to_json             # => "{\"name\":null}"
+
+    class SerialPerson
+      include ActiveModel::Serializers::Xml
+    end
+
+    s = SerialPerson.new
     s.to_xml              # => "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<serial-person...
 
   {Learn more}[link:classes/ActiveModel/Serialization.html]