Add Zenity as a noted requirement for Linux

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.