New maintainer for XDG specs

Hi all,
After a bit of a period of inactivity, Matthias Klumpp has agreed to help update and maintain the XDG specs, including the menu and icon-theme specs in particular.

If you have any suggestions or anything you want addressed, please discuss it on the list, or file an issue or merge request.

Thanks to Matthias for stepping up, as well as to Bastien Nocera and David Faure in particular for all their work over the years to get us to this point.

Cheers,
Daniel