Minor edits to README

Suggestions for minor additions to the README that would have saved me some time. 

The hint on defining rules in an initializer is somewhat obvious but adds to the installation instructions. 

Also, I think that when working to integrate Rack::Attack into an app, it's easy to forget that throttling requires the cache. So I think there is merit in saying that the cache needs to be turned on in dev mode.

Thanks - I hope you find these suggestions useful.