add missing versionadded directive

Author: xabbuh

cookbook/email/email.rst

@@ -138,6 +138,11 @@ template might look something like this:
     {# Makes an absolute URL to the /images/logo.png file #}
     <img src="{{ absolute_url(asset('images/logo.png')) }}"
 
+.. versionadded:: 2.7
+    Support for creating absolute URLs with the ``asset()`` function will
+    be dropped in Symfony 3.0. Starting with Symfony 2.7, you should use
+    the ``absolute_url()`` function instead.
+
 The ``$message`` object supports many more options, such as including attachments,
 adding HTML content, and much more. Fortunately, Swift Mailer covers the topic
 of `Creating Messages`_ in great detail in its documentation.