Add info about AUR package (Arch-based Linux distributions)

I liked KeyboardVisualizer so I made a AUR package to make it a bit quicker to install and more discoverable for people using Linux distros based on Arch. (Package link: https://aur.archlinux.org/packages/keyboard-visualizer-git/)

This PR basically adds information to the Readme about the AUR package and how to install it. (Also fixes the icon URL at the top of the readme)