Ensure people know what the memcache setting does

Even though memcache can be disabled, the memcache library is a
requirement as listed in the README.md. I clarified that in the
configuration file itself to ensure people install it even though they
might be disabling it for debugging purpose.

Fixes #409
Merge pull request #414 from TheSerapher/issue-409

Ensure people know what the memcache setting does