The script takes one large PNG image and creates AppIcon.appiconset with all icons prefilled.
Image must be square and not less than 1024 x 1024 px.
The script does not only creates and fills images for an Xcode project, the AppIcon folder it provides is also smaller compared to the one filled within Xcode, because nipple links the same file for containers of the same size instead of copying the file as Xcode does.
/path/to/nipple /path/to/your/largest_icon.png
And that's it. A folder will appear near the specified image.
Now AppIcon.appiconset folder in your project (within Assets.xcassets folder) can be simply replaced with the one that was created by script. Enjoy!