Document ONBOARDING process (also create onboarding process)

[gibfahn]

Refs: https://github.com/nodejs/LTS/pull/223/files#r127595396

It's probably time we tried to write down some more stuff about what happens if you join the team, especially now that it means joining the Release WG as well.

As an aside, (cc/ @MylesBorins) this will mean that we have to be somewhat more careful about adding new CitGM collaborators.