[Buildbot] #3406: Create wiki page for external plugins, link to it from docs

[Buildbot trac]

#3406: Create wiki page for external plugins, link to it from docs
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Reporter:  dustin       |       Owner:  dustin
    Type:  enhancement  |      Status:  new
Priority:  minor        |   Milestone:  0.9.0
 Version:  master       |  Resolution:
Keywords:               |
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Comment (by dustin):

 https://github.com/buildbot/buildbot/pull/2000

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Ticket URL: <http://trac.buildbot.net/ticket/3406#comment:1>
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