[symfony#5761] add versionadded directive

Author: xabbuh

cookbook/security/remember_me.rst

@@ -83,6 +83,9 @@ the session lasts using a cookie with the ``remember_me`` firewall option:
 The ``remember_me`` firewall defines the following configuration options:
 
 ``secret`` (**required**)
+    .. versionadded:: 2.8
+        Prior to Symfony 2.8, the ``secret`` option was named ``key``.
+
     The value used to encrypt the cookie's content. It's common to use the
     ``secret`` value defined in the ``app/config/parameters.yml`` file.