[1.6.x] Fixed #20938 -- Added cached sessions note to deployment checklist.

Thanks mjtamlyn for the suggestion.

Backport of 4e784f337c from master

docs/howto/deployment/checklist.txt

@@ -87,6 +87,9 @@ and in production.
 Cache servers often have weak authentication. Make sure they only accept
 connections from your application servers.
 
+If you're using Memcached, consider using :ref:`cached sessions
+<cached-sessions-backend>` to improve performance.
+
 :setting:`DATABASES`
 --------------------