doc: add iarna to collaborators

PR-URL: https://github.com/nodejs/node/pull/4144


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>
Reviewed-By: Kat Marchán <kzm@sykosomatic.org>
Reviewed-By: James M Snell <jasnell@gmail.com>