Included extend() explanation in services documentation chapter

Together with the Twig add global and filter example.

doc/services.rst

@@ -161,6 +161,31 @@ using the ``protect`` method::
 
 Note that protected closures do not get access to the container.
 
+Modify services after definition
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Sometimes you want to alter a service after its definition. Pimple facilitates
+this by ``extend``-ing the already defined service.
+
+First argument of the ``extend`` method is the name of the service you want to
+modify. Second argument is a callable. This callable is executed with the service
+you want to alter as its first argument, the service container self is provided
+in the second argument.
+
+.. note::
+
+    Be sure to return the modified service in the callable.
+
+You can use this pattern to add functionality to :doc:Twig <providers/twig> for
+example::
+
+    $app->extend('twig', function($twig, $app) {
+        $twig->addGlobal('pi', 3.14);
+        $twig->addFilter('levenshtein', new \Twig_Filter_Function('levenshtein'));
+        
+        return $twig;
+    });
+
 Core services
 -------------