Create doc and SystemD-files to run Prax on Arch

[tijn]

This pull request replaces the previous one https://github.com/ysbaddaden/prax.cr/pull/22.

Note that I made a PKGBUILD in a separate repository that you can find here: https://github.com/tijn/prax-arch . You may copy the PKGBUILD into the Prax repository but it for an AUR package we need to have a separate repository since the file .SRCINFO needs to be present in all commits. See https://wiki.archlinux.org/index.php/Arch_User_Repository#Uploading_packages