Add documentation about impersonating users with an HTTP Header

Author: srogier

security/impersonating_user.rst

@@ -84,6 +84,8 @@ To switch back to the original user, use the special ``_exit`` username:
 
     http://example.com/somewhere?_switch_user=_exit
 
+You can also send the ``_switch_user`` parameter as an HTTP header in your request.
+
 During impersonation, the user is provided with a special role called
 ``ROLE_PREVIOUS_ADMIN``. In a template, for instance, this role can be used
 to show a link to exit impersonation: