Better docs about icons

[pomali]

Had quite a lot debugging around using icons on macOS.

Problem 1

there is no documentation that you need to edit assets/dune (in revery-quick-start repo)
like

(install
    (section bin)
    (package App)
-   (files Roboto-Regular.ttf))
+   (files Roboto-Regular.ttf icon.icns))

Problem 2

documentation uses
"iconFile": "assets/icon.icns"
this doesn't correspond with what gets created when you add icon like I did in dune,
and leads person to expect their assets dir is used, but this is path inside *.App package. Even now I'm not sure if it is path inside Contents/MacOS/ or Conents/Resources but they are symlinked so it probably doesn't matter.

All in all little more docs would help here :)