doc: add notable-change to onboarding.md exercise

Pull requests that add new Collaborators should be labeled
`notable-change`. Make a note of this in onboarding.md.

PR-URL: https://github.com/nodejs/node/pull/21040


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>