Do a quick dependency check before running install script (+ add notes in DEPENDENCY.md for NixOS users)

[bayazidbh]

Was just installing the pro/itch.io version on a new NixOS setup.

It seems that by default NixOS lacks file command and the xdg-utils and desktop-file-utils packages.

Should probably do a check for those before starting the rest of the scripts -- and adding a section on DEPENDENCY.md file on the github page would also help.

Edit: oh, right - NixOS don't use FHS so there's some issues with stub-ld. It requires nix-ld Option there (in addition to envfs to run the script as that calls /bin/bash on shebang).

[bakustarver]

I am not familiar with NixOS's dependencies. Feel free to publish your results.