Make sure /etc/portage/repos.conf directory exists and git installed.
curl https://raw.githubusercontent.com/aaaaadrien/adrien-overlay/master/repo.conf -o /etc/portage/repos.conf/adrien-overlay.conf
curl https://raw.githubusercontent.com/aaaaadrien/adrien-overlay/master/repositories.xml -o /etc/layman/overlays/adrien-overlay.xml
layman -S
layman -a adrien-overlay