Add RethinkDB Engine link

[Kingsley Hendrickse]

Hi

I've written an engine adaptor for RethinkDB - would you mind adding it to the list of available engines on your README.

The repo is here: https://github.com/kingsleyh/kemal-session-rethinkdb

Also if you see any issues with it please let me know. I followed the same pattern as @crisward used for his kemal-session-mysql adaptor.

Thanks

--Kingsley