Include category in the desktop file

[nedrichards]

the .desktop file for Linux doesn't specify any categories - which means that Zotero won't show up by default in a number of Linux software stores such as GNOME Software. A quick:

Categories=Education;

Would probably be fine, but you may have different views on the correct value.

[arteymix]

It fits more into the Office category considering that the main purpose is to collect, manage and cite documents. It's not exclusive to scientific research.

[nedrichards]

Fair enough, I've pushed an update to the PR which says "Office" rather than "Education".