docs: record the coupling with qtmaildirHEADmaster

mailrules.py and qtmaildir's src/tagrules.cpp are two implementations of
one format, so a schema change is a two-repo change: a field added on one
side is silently dropped when the other saves.

Also notes that post-new is installed and live, so a change here reaches
real mail on the cron timer rather than when someone next runs a command.

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