doc: add imyller to collaborators

Signed-off-by: Ilkka Myller <ilkka.myller@nodefield.com>

PR-URL: https://github.com/nodejs/node/pull/8530


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>