Add Crystal client

[gabor-boros]

https://spectrum.chat/rethinkdb/general/crystal-developers-could-use-the-driver-updated-by-kingsleyh~5b4b7142-15af-4784-b45d-9c6f86120dd3

 Christian Kakesa:
https://github.com/kingsleyh/crystal-rethinkdb
It could be awesome that this driver become an official RethinkDB drivers.

Sam:
It would make sense to add a link to https://rethinkdb.com/docs/install-drivers/ or some other website page.