[Buildbot] #3442: document usage of IPlugin subclasses

[Buildbot trac]

#3442: document usage of IPlugin subclasses
----------------------+-------------------
Reporter:  rutsky     |      Owner:
    Type:  undecided  |     Status:  new
Priority:  major      |  Milestone:  0.9.+
 Version:  master     |   Keywords:
----------------------+-------------------
 From https://github.com/buildbot/buildbot/pull/1961/files


 All plugins kinds are listed in [http://docs.buildbot.net/latest/developer
 /plugins-publish.html#making-the-plugin-package docs]:

 {{{
 Buildbot supports several kinds of pluggable components:

 * ``buildslave``
 * ``changes``
 * ``schedulers``
 * ``steps``
 * ``status``
 * ``util``
 }}}

 {{{
     setup(
         ...
         entry_points = {
             ...,
             'buildbot.kind': [
                 'PluginName = PluginModule:PluginClass'
             ]
         },
         ...
     )
 }}}

 but no information is given what is `PluginClass`: it must be `IPlugin`
 subclass, and more precisely must be subclass of kind interface, e.g.
 `IChangeSource` for plugins in `changes` entry point.

--
Ticket URL: <http://trac.buildbot.net/ticket/3442>
Buildbot <http://buildbot.net/>
Buildbot: build/test automation