[Buildbot] #3355: Create a nine migration guide

[Buildbot trac]

#3355: Create a nine migration guide
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Reporter:  dustin       |      Owner:  dustin
    Type:  enhancement  |     Status:  new
Priority:  critical     |  Milestone:  0.9.0
 Version:               |   Keywords:  docs
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 The docs should have a migration guide.

 However, this will need to evolve more quickly than releases.  So the bulk
 of the guide should be in a wiki format, so it can be edited easily.  The
 official docs should link there.

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Ticket URL: <http://trac.buildbot.net/ticket/3355>
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