Mention usage of generational cache key components in README.

I found it useful to be able to invalidate a the cache result of a `cache_or_304` block independent of the bindings (e.g., when logic underlying the block's result is updated).

Thanks @macreery for pointing me in the right direction.

Nicely formatted view of the full file with changes is [here](https://github.com/joeyAghion/garner/blob/4b6c6b17e955d6aab74534f42112566f7fe5abd8/README.md#fetching-objects-directly-from-cache).