doc: encourage 2FA before onboarding

In the onboarding document, add a note to ask the new Collaborator if
they are using two-factor authentication on their GitHub account. If
they are not, suggest that they enable it as their account will have
elevated privileges in many of the Node.js repositories.

PR-URL: https://github.com/nodejs/node/pull/8776
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Johan Bergstrom <bugs@bergstroem.nu>
Reviewed-By: Robert Jefe Lindstaedt <robert.lindstaedt@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

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