Add 0.4 what's new section about coordinates

As discussed in #2416, this adds a "what's new" section about the APE5 coordinate changes.

As also discussed in #2416, I'm about to begin some travel and am not sure when I'll next have good internet.  So if there are comments that require changes and I'm unresponsive,  @taldcroft, feel free to make your own PR based on this and merge that instead of this.

cc @astrofrog @Cadair @adrn
"not-built-in" -> custom?