Thanks, it is actually there on my local install when I generated the package file but it is indeed not on Github itself as "hidden" files starting with dot are ignored. That somehow escaped me (I really don't check all the files TinyMCE contains…). The file itself is not important though as it appears to be a left over from its release process.
You can get rid of the warning by re-creating the file/folder package to check yourself using this dev plugin:
https://github.com/zenphoto/DevTools/tree/master/plugins/zenphoto_package