[Buildbot] #2945: Add an example of using codebases

[Buildbot trac]

#2945: Add an example of using codebases
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Reporter:  sa2ajj            |      Owner:
    Type:  enhancement       |     Status:  new
Priority:  patches-accepted  |  Milestone:  0.9.+
 Version:  master            |   Keywords:  docs
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 While the notion is introduced and all relevant options are documented, it
 seems to be a good idea to add a separate example where codebases are
 used.

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Ticket URL: <http://trac.buildbot.net/ticket/2945>
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