Update readme with instructions for releasing old release branches (#1829)
Update readme with instructions for releasing old release branches (#1829)

* Update readme with instructions for releasing old release branches

* Apply suggestions from code review

Co-Authored-By: Matt Moore <866b498749d560dfd4b9805e44d1fa0b8bc948c7@vmware.com>

Co-authored-by: Matt Moore <866b498749d560dfd4b9805e44d1fa0b8bc948c7@vmware.com>
knative/test-infra,knative/test-infra,knative/test-infra