zmailer install docs

[Arnt Gulbrandsen <agulbra@troll.no>]

The zmailer install docs should mention the 'zmailer' group and its
default, and the sample aliases file ought possibly to mention
draft-vixie-ops-stdaddr-*.txt:

   Abstract

      This draft enumerates and describes standard electronic mail
      addresses to be used when contacting the operations personnel of an
      arbitrary domain.

      As an operational standard, the recommendations herein pertain to
      vendors only inasmuch as their end user documentation should
      recommend that these mail addresses be aliased to appropriate end
      user personnel.

      This document should be advanced as a Best Current Practice, since it
      describes what the current practice is and should be.

--Arnt