Create subteams to document release/infra/github-bot access

Creating a separate issue to propose adding these teams (otherwise it'll slip through the cracks).

Currently #811 just has lists of names, which I assume won't get updated.

Agreement in the WG meeting was to document this in the Readme, and have the access request process be the same as it is for general Build WG access, a PR to the Readme adding one's name.