doc: clarify part of onboarding guide regarding adding to teams

There are a number of teams (for example, the build team) which require more than just showing interest as a prerequisite for being added, so this change adds a link to the set of subsystem teams where new Collaborators can be added directy.

Also, the doc didn't actually mention if the new Collaborator should be added to the team. This change also clarifies that.

Refs: https://github.com/nodejs/build/issues/3003 Signed-off-by: Darshan Sen raisinten@gmail.com