[#1958] Adding versionadded note

Author: weaverryan

book/forms.rst

@@ -190,6 +190,9 @@ it into a format that's suitable for being rendered in an HTML form.
    (e.g. ``isPublished()`` or ``hasReminder()``) instead of a getter (e.g.
    ``getPublished()`` or ``getReminder()``).
 
+   .. versionadded:: 2.1
+        Support for "hasser" methods was added in Symfony 2.1.
+
 .. index::
   single: Forms; Handling form submission