Document how to get custom fonts to work when targeting macOS

[dfabulich]

On macOS, you have to set ATSApplicationFontsPath.

https://developer.apple.com/documentation/BundleResources/Information-Property-List/ATSApplicationFontsPath

If you set this key, the system allows the app in the bundle to use the fonts at the specified path. Set this key to the path relative to the bundle’s Resources folder. For example, if the fonts are in .../Resources/MyFonts, set this key to MyFonts/.

It seems to work for me to set it to ./. I applied this change to the Showcase app in https://github.com/skiptools/skipapp-showcase/pull/23 and it fixed custom fonts in that app.