docs(app): add installation instructions using AUR

[edbar42]

better-commits is also available as an AUR on https://aur.archlinux.org/packages/better-commits

I took the liberty of adding it to the installation instructions since AURs are easier to integrate with stuff like installation scripts on Arch Linux.

Btw, thanks for making better-commits. Great tool all around.

[Everduin94]

Hey thanks for taking the time to open this PR! - I do want to add the Arch install to the docs. @tonitch has maintained the project there for a long time, I think it makes sense to add it.

Ideally, we could close this first - #52. That way we could have 1 section like this:

📦 Installation

better-commits can be installed with a variety of package managers:

# NPM
npm install -g better-commits

# AUR
yay -S better-commits

# Homebrew
brew install better-commits