For now, this overlay is not part of layman so registration has to be done manually.
$ git clone https://github.com/shnups/gentoo-overlay.git /path/to/local/clone/shnups-overlay
[shnups]
priority = 50
location = /path/to/local/clone/shnups-overlay
sync-type = git
sync-uri = https://github.com/shnups/gentoo-overlay.git
eselect repository enable shnups