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.
I would change ``how to plot data  using the astropy.coordinates package in the Aitoff projection`` to ``how to make a plot in the Aitoff projection using data in a `SkyCoord` object.``  `astropy.coordinates` includes other objects that *could* be used to do this, but in this particular case you're using `SkyCoord`.