Add arch linux install to the README

[williamjameshandley]

Excellent CLI & REPL!

I've added this to the arch linux AUR, so if you wish to you could add these installation instructions to the README: Arch linux users can install from the [AUR](https://aur.archlinux.org/packages/python-gpt-command-line)

[emilrueh]

I have just attempted to install your AUR package by running yay -S python-gpt-command-line but am getting:

-> No AUR package found for python-google-ai-generativelanguage=0.6.9 -> could not find all required packages: python-google-ai-generativelanguage =0.6.9

[williamjameshandley]

I believe this should have been fixed -- if you run yay -Syu python-gpt-command-line, possibly after removing/uninstalling the relevant ~/.cache/yay/ this should work.