shared/contrib-additional.adoc: Add Stephan Lichtenauer

Stephan works on the third party project potluck
(https://github.com/bsdpot/potluck) and has reported about pot and
potluck in the FreeBSD Status reports.

His first authored commit is in the doc repository:
eb2e55d843695eb0c5b4d3f289a72f6b07543581

But he had already contributed with status reports before, to the now
archived https://github.com/freebsd/freebsd-quarterly repository.

Approved by: carlavilla (mentor)
Differential Revision: https://reviews.freebsd.org/D39361