This is a custom repository of packages (similar to Arch Repositories, extra, community, etc.)
To install/update packages from this repo, add these lines to your /etc/pacman.conf
:
[adig-pkgs]
SigLevel = Optional
Server = https://github.com/adig-pkgs/$repo/raw/gh-pages/$arch
Then just use pacman, as you normally do, for eg. pacman -Sy worldlinesim-git