Add Marcel Kaiser to contributors

He is the author and maintainer of these ports:
audio/dsbmixer
sysutils/dsbbatmon
sysutils/dsbdriverd
sysutils/dsbmc
sysutils/dsbmd
sysutils/dsbwrtsysctl

Approved by: garga (mentor)
Differential Revision: https://reviews.freebsd.org/D10965