Add Arch Linux install instructions for official package

Hi Greg!

AUR-Maintainer of yaak-bin, yaak-git, yaak-appimage, and formerly (!) the yaak package here.
Some exciting news you might have missed: The yaak package has been removed from the AUR and officialy added to the Arch Linux [extra] repository, meaning it’s now out of my hands, and in the hands of an approved official package maintainer and distributed much more securely and with plenty of mirrors around the world!
I think this makes it one of the first official Linux package repositories to have a yaak package?

So to honor this, I thought it would be time to add installation instructions to the Yaak docs for installing it on Arch Linux with:

pacman -S yaak

Here’s the link to the package: https://archlinux.org/packages/extra/x86_64/yaak/

Let me know what you think!
~ Jonas