doc: add optional step to onboarding doc

Include information on how to force push to the contributor's own branch
so that pull requests show as merged in GitHub interface.

PR-URL: https://github.com/nodejs/node/pull/8774


Reviewed-By: Robert Jefe Lindstaedt <robert.lindstaedt@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Johan Bergstrom <bugs@bergstroem.nu>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: James M Snell <jasnell@gmail.com>