what file inside?
Create *.r2skin-addon file is almost same with that create *.r2skin file, there only a few different, eg:
a) Add a no extension text file - "skin", its content is sikn's name and version. (before b131, version number must fully same)
For example: if you want to add into your newest skin ChromophoreLou, then the content is ChromophoreLou
b) Rename file skin.xml to widget-addon.xml, and change the content <include file="xml/elements.xml" /> to <include file="xml/widget-elements.xml" />
c) Rename file 'xml/elements.xml' to 'xml/widget-elements.xml'.
Note: if not do b) c), will (can) overwrite the old one.
You can download following file to see, it can add/overwrite weather window into shadow4.