binary option is undocumented

[lindes]

I was just helping someone try to set up express-redis-cache for a web app they're developing that uses an API to do text-to-speech and generate mp3 files. The API has rate limiting, so we wanted to set up caching. This seemed like a perfect solution, and indeed it works well -- once we knew we needed the binary option in our cache.route call.

Could you please add something to the main readme that mentions the option, as defined here and tested here?

Thanks.

(And sorry, I'd send a pull request, but I'm off to help someone else with a different thing.)