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.
Instead of doing this as shown in my original example, I think you can now do (above):

```
ra_rad = c.ra.wrap_angle(180 * u.deg).radian
```

and that will deal with the wrapping. Could you update to use this instead? Just push to the same branch, no need to open a new pull request.