Add an example to the invokable controllers section

Author: javiereguiluz

controller/service.rst

@@ -80,7 +80,22 @@ Invokable Controllers
 
 If your controller implements the ``__invoke()`` method - popular with the
 Action-Domain-Response (ADR) pattern, you can simply refer to the service id
-(``AppBundle\Controller\HelloController`` or ``app.hello_controller`` for example).
+(e.g. ``AppBundle\Controller\HelloController`` or ``app.hello_controller``)::
+
+    // src/AppBundle/Controller/Hello.php
+    use Symfony\Component\HttpFoundation\Response;
+    use Symfony\Component\Routing\Annotation\Route;
+
+    /**
+     * @Route("/hello/{name}", name="hello", service="AppBundle\Controller\Hello")
+     */
+    class Hello
+    {
+        public function __invoke($name = 'World')
+        {
+            return new Response(sprintf('Hello %s!', $name));
+        }
+    }
 
 Alternatives to base Controller Methods
 ---------------------------------------