document maintenance of kubernetes-milestone-maintainers GH team

[tpepper]

Who should be added to and removed from the kubernetes-milestone-maintainers GH team, and when, needs documented.

https://github.com/kubernetes/sig-release/blob/1b389936697e0881dfc343f634da2d37f03604ff/release-team/role-handbooks/ci-signal/README.md previously referred to but that and https://github.com/kubernetes/community/blob/master/project-managers/README.md#joining-the-group now 404's as #2357 removed it. The documentation there perhaps was more about joining the kubernetes-pm team, but should have been superseded by a process for kubernetes-milestone-maintainers.

It's now possible (see kubernetes/test-infra@19b225c) for repos to have their own milestone maintainers lists, but kubernetes-milestone-maintainers remains the default and is managed in an ad hoc by SIG-Release and the release team lead. But that's likely excessive in most cases.

Probably a simple process here works fine if it’s a limited number of people per sig/repo and we have a simple process like sig lead tells release lead who to add/remove…something light weight could work just fine I reckon. Only if sigs are rapidly adding/removing lots of people, then probably better for them to manage their own milestone team.

[tpepper]

/cc @spiffxp @jberkus @jdumars @justaugustus @idvoretskyi

[tpepper]

/sig pm
/sig release

[justaugustus]

Great minds, @tpepper: kubernetes/enhancements#590 (comment)

[justaugustus]

RFC for milestone-maintainers notes: kubernetes/sig-release#239

[justaugustus]

/assign

[justaugustus]

Added in kubernetes/sig-release#239
/close