Add Arch Linux install instructions

[ghost]

No description provided.

[brunswick42]

I think this would be a worthwhile addition. There are currently three deb based installation instructions and two rpm based instructions, but nothing for pacman based distributions.

While I'd be interested in adding the instructions, I'm not sure which approach is best. Unlike Ubuntu/Debian/CentOS etc. there is no Mono repository for Arch Linux. The Arch Linux Mono package is up-to-date with the latest stable release, but there are no preview or nightly releases.

[ghost]

I found links for mono beta/nightly beta nightly

[brunswick42]

Thanks for pointing those out. I didn't look at the AUR. I thought beta was distinct from preview releases, though I am new to mono and may be wrong about that. However, the mono-beta package was last updated on 2018-02-19, so it is quite out of date. Likewise, mono-nightly was last updated with 2020-06-08, with mono-6.13.0.588.