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

[Buildbot trac]

#2945: Add an example of using codebases
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Reporter:  sa2ajj            |       Owner:  Ben
    Type:  enhancement       |      Status:  assigned
Priority:  patches-accepted  |   Milestone:  0.9.+
 Version:  master            |  Resolution:
Keywords:  docs              |
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Comment (by sa2ajj):

 Things to cover:

 * codebase generator
 * scheduler configuration
 * change filter
 * use of codebase in build steps

 Ben, an example where it makes sense is a two repo setup (e.g. lib + app)
 where we want to build both regardless of whether the change is coming to
 the lib repo or to the app repo.  And in this scenario we'd like to build
 the revision for the change + master/stable of the other repo.

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Ticket URL: <http://trac.buildbot.net/ticket/2945#comment:4>
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