Add install instructions for Arch Linux

[skybert]

I've verified that the version installed on Arch is indeed coming from the jtesta fork:

    [root@2e9f9f9f8cdf /]# grep PRETTY /etc/os-release
    PRETTY_NAME="Arch Linux"
    [root@2e9f9f9f8cdf /]# ssh-audit --version | head -n 1
    # ssh-audit v2.5.0, https://github.com/jtesta/ssh-audit

I've verified that it works on Garuda Linux too (and probably other Arch derivatives as well).