Document Default Team Permissions

[romeara]

Add documentation of the default permission sets, and current implementation of:

• Admin: Organization Admins
• Write: The Vanguard
• Triage: The Calvary

[romeara]

As part of this, might be good to also define the process for moving people between teams - criteria, how to request, etc (open an issue in this repo if you think someone should be moved?)