[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       |   Keywords:
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 Starting with
   https://github.com/buildbot/buildbot/pull/1948#issuecomment-170612262

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