a reminder to PR submitters

[Sergey Matveychuk]

Adam Weinberger wrote:
> Submitter-Id
> 	"current-users" doesn't mean a thing. Submitter-Id should be
> 	usable as "user" or "maintainer," depending upon who submits the
> 	PR. If a maintainer wishes to submit a PR for an update to a
> 	port she maintains, she should be able to choose
> 	Class:update/upgrade, and Submitter-Id:maintainer.

It's a something new for me. And I think must be documented somewhere.

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Sem.