Add new contributors:

Andris Raugulis for sysutils/s6-rc
Robert Zelaya for devel/oniguruma6
Mahdi Mokhtari for databases/mysql80-server
Steffen Fritz for sysutils/siegfried
Zsolt Udvari for x11-wm/wmname

Reviewed by: matthew
Approved by: matthew (mentor)
Differential Revision: https://reviews.freebsd.org/D9148