Add docs about caching

Re: https://github.com/rails-api/active_model_serializers/pull/89, https://github.com/rails-api/active_model_serializers/issues/302, and https://github.com/rails-api/active_model_serializers/pull/343

This feature has been in for months, and is being used in production (by us, at least!).  It should really probably be documented.  I was initially unaware that a `cache_key` method was mandatory (not just falling back to `object.cache_key`), and some docs would have saved me a lot of time.