How to add an app icon for your action extension?

The documentation doesn’t explain how. The Human Interface Guidelines states it should be a template image centered in about a 70pt asset. People on Stack Overflow are suggesting adding an AppIcon in the assets catalog, putting them in the 60pt iPhone App slots, and going to the Build Settings and setting Asset Catalog App Icon Set Name to AppIcon. That works… even on iPad, but it’s not 70x70 and you get warnings about not having 76, 83.5, and 1024pt app icons. :/

I filed FB7850720 Include Action Extension app icon asset and/or document how to add one