doc: update onboarding task

Use fast-tracking on PRs where new Collaborators are adding themselves
to the README.

PR-URL: https://github.com/nodejs/node/pull/23300


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>