doc: add joaocgreis as a collaborator

Refer: https://github.com/nodejs/io.js/issues/2102

PR-URL: https://github.com/nodejs/io.js/pull/2208



Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>