Add Zenity as a noted requirement for Linux

[rm-steele]

I was attempting to play Ship on a fresh Arch Linux install of the game, and was getting the AppImage closing in less than a second with not so much as an error printed to the terminal. After checking through all the setup guides I could find, I wasn't able to find anything more than "yeah you just execute the appimage bro". After searching the issues for a bit I found #5894 and sure enough that was the problem.

I'd like there to be some sort of indication of this requirement, either in an error message or noted in the setup guides.

[Malkierian]

We're trying to work on processes that remove the requirement. We know we didn't realize it at first because Zenity is also installed by Steam, and it wasn't until people with Linux installs that didn't have Steam installed that we started discovering that requirement. Current develop mostly has that handled, I think, but it might still be required even just for the file select dialogs.

[rm-steele]

Ah cool, that would work as well