Example of plotting coordinates on an all-sky map (final pull request)

This should be the final pull request as discussed in issue #2726. There were some problems with my repo, so I had to re-create my branch and so github doesn't allow me to update the original pull request.
maybe make the references to numpy and matplotlib link to the relevant web sites?  E.g., `` `numpy 
<http://www.numpy.org/>`_ `` and  `` `matplotlib 
<http://www.matplotlib.org/>`_ ``.

Alternatively, you can add these links to the ``references.txt`` file in the coordinates doc folder.  then you can just reference them as `` numpy_ `` and ``matplotlib_``