Document example redis cache usage with custom redis configuration

[javorszky]

Context: suborbital/e2core#165 (comment)

This is more of a quirk on how to use redis as a cache and not strictly using atmo, but because caching is part of our runnables ecosystem, it might be a great idea to reduce barrier to entry by documenting somewhat advanced implementations of different caches.

In redis's case, the answer to @k33g's question could added to an advanced tutorials / advanced usage guide / example page.