added the example Twig Code for the HTML5 no novalidate attribute

in the form documentation, there is a notice about the no validate HTML5 attribute, but without the twig example.

I added the twig code

Author: sebastianblum

book/forms.rst

@@ -420,6 +420,11 @@ corresponding errors printed out with the form.
    but are being prevented by your browser from, for example, submitting
    blank fields.
 
+    .. code-block:: html+jinja
+
+       {# src/Acme/DemoBundle/Resources/views/Default/new.html.twig #}
+       {{ form(form, { 'attr': {'novalidate': 'novalidate' }}) }}
+
 Validation is a very powerful feature of Symfony2 and has its own
 :doc:`dedicated chapter </book/validation>`.