Add through-cache function.

[Luke1298]

It looks like this "through-cache" pattern was introduced some what recently: https://github.com/clojure/core.cache/blame/5b8474a532a7f12ca1a1fbbf8f82a6f816c16b29/src/main/clojure/clojure/core/cache.clj#L57

Since the readme references these docs which mention it I figured it'd be helpful to include.