doc: mention git-node in the collaborator guide

PR-URL: https://github.com/nodejs/node/pull/18960
Fixes: https://github.com/nodejs/node/issues/18197


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Matheus Marchini <matheus@sthima.com>
Reviewed-By: Shingo Inoue <leko.noor@gmail.com>