Local overlays should be managed via /etc/portage/repos.conf/.
To enable this overlay create a /etc/portage/repos.conf/kressh.conf file:
[kressh]
location = /var/db/repos/kressh
sync-type = git
sync-uri = https://github.com/kressh/gentoo-repo.git
Afterwards, simply run emerge --sync kressh, and Portage should seamlessly make all our ebuilds available.