add reference to mongodb session storage to cookbook index

Author: stevenmusumeche

cookbook/map.rst.inc

@@ -188,6 +188,7 @@
   * :doc:`/cookbook/session/sessions_directory`
   * :doc:`/cookbook/session/php_bridge`
   * (configuration) :doc:`/cookbook/configuration/pdo_session_storage`
+  * (configuration) :doc:`/cookbook/configuration/mongodb_session_storage`
   * :doc:`/cookbook/session/avoid_session_start`
 
 * **symfony1**