[MacPorts] #55448: Document github portgroup in guide

[MacPorts]

#55448: Document github portgroup in guide
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  Reporter:  emcrisostomo  |      Owner:
      Type:  defect        |     Status:  new
  Priority:  Normal        |  Milestone:
 Component:  guide         |    Version:
Resolution:                |   Keywords:
      Port:                |
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Comment (by emcrisostomo):

 Thanks you very much Ryan.

 I've created the following pull request (https://github.com/macports
 /macports-guide/pull/12) in order to contribute documentation of this
 portgroup to the macports guide. As you'll see, I've heavily used the
 documentation in the portgroup file itself. Since English is not my native
 language, please feel free to provide any feedback that you consider
 appropriate.

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Ticket URL: <https://trac.macports.org/ticket/55448#comment:2>
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