[MacPorts] #56056: Document milestone field handling for base tickets

[MacPorts]

#56056: Document milestone field handling for base tickets
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 Reporter:  neverpanic   |      Owner:  neverpanic
     Type:  enhancement  |     Status:  new
 Priority:  Normal       |  Milestone:
Component:  guide        |    Version:
 Keywords:               |       Port:
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 wiki:Meetings/MacPortsMeeting2018/Roadmap now documents how we should set
 the milestone field when fixing or planning tickets. This wiki page is not
 the correct place for this documentation, it should be moved to the guide.

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Ticket URL: <https://trac.macports.org/ticket/56056>
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