Add a link to Multiple User Providers

Author: thePanz

book/security.rst

@@ -358,6 +358,11 @@ probably only need one. If you *do* have multiple, you can configure which
 *one* provider to use for your firewall under its ``provider`` key (e.g.
 ``provider: in_memory``).
 
+.. seealso::
+
+    See :doc:`/cookbook/security/multiple_user_providers` for
+    all the details about multiple providers setup.
+
 Try to login using username ``admin`` and password ``kitten``. You should
 see an error!