governance.md: add inactivity policy for any roles

[bjohansebas]

The goal of this is to have something written for when a member is considered inactive and to establish a certain process. Any suggestions on the wording are welcome.

This is important because of roles like captains who have access to package publishing—having someone with access who is no longer active in the project can be risky, so only active people should have access

[wesleytodd]

Just small changes, but I think we need to figure out the 6mo vs 12mo thing before we merge, hence the "request changes" and not just "approve".

[wesleytodd]

One nit, but otherwise LGTM.