Add some examples to github page

Release 0.7.0

I've updated the readme to clarify caches are immutable and to show suggested usage with atom/swap! (note: uses through-cache which is also new in the upcoming release – the current through function does not play well with swap! etc).

@alexmiller
Attaching a diff with a note about cache state and and an example.

I've used this library a few times, and each time I have to re-learn that the cache is not stateful. I think it would be really helpful if this were clearly called out in the README and a trivial example supplied.

Can do. In the meantime checkout how core.memoize uses it at https://github.com/clojure/core.memoize/blob/master/src/main/clojure/clojure/core/memoize.clj#L50

I think I might bring this through fn into core.cache in a more generic way.