SEP for the re-implementation of `sunpy.wcs` as `sunpy.coordinates`

Putting in the PR so that people can form their opinions on what I've written so far.
Fire away.
I would re-write this whole sentence. Say that the SunPy coordinates package is going to use the Astropy coordinates package and that it is specified in APE 5, (also provide a link to the main astropy docs in the reference section) and then describe how the *SunPy* coordinates package will be laid out, which is the same as describing the astropy one, just more focused on the bits we are using.