add xml example to lock protection cookbook

Resources/doc/cookbook/recipe_lock_protection.rst

@@ -32,6 +32,8 @@ You can enable it in your ``sonata_admin`` configuration :
                 
 You must also configure each entity that you want to support by adding a field called ``$version`` on which the Doctrine ``Version`` feature is activated.
 
+Using Annotations:
+
 .. code-block:: php
 
     <?php
@@ -53,6 +55,23 @@ You must also configure each entity that you want to support by adding a field c
         // ...
     }
 
+Using XML:
+
+.. code-block:: xml
+
+    <?xml version="1.0" encoding="utf-8"?>
+    <!-- src/AppBundle/Resources/orm/Car.orm.xml -->
+    <doctrine-mapping>
+        <entity name="AppBundle\Entity\Car">
+            <!-- ... -->
+    
+            <field name="version" type="integer" version="true" />
+    
+            <!-- ... -->
+        </entity>
+    </doctrine-mapping>
+    
+
 For more information about this visit the `Doctrine docs <http://doctrine-orm.readthedocs.org/en/latest/reference/transactions-and-concurrency.html?highlight=optimistic#optimistic-locking>`_
 
 .. note::