Instructions: Forgets to mention requiring zenity

[Ruedii]

Describe the bug The instructions fail to mention that Zenity needs to be installed to run Fails to run with --gui as a result giving cryptic python crash error.

To Reproduce Steps to reproduce the behavior:

1. Install ProtonTricks by instructions provided in README.md on a fresh Kubuntu installation
2. Attempt to run with the --gui option

Expected behavior The instructions should include Zenity in the needed packages, or the program should have a proper error for this case.

System (please complete the following information):

• Distro: Kubuntu 19.04.1
• Protontricks version: protontricks (1.4.3)
• Steam version: current standard (not beta)

[Matoking]

Thanks, I've updated the README and made the error message clearer.