shared/contrib-additional.adoc: Add Mina Galić

Mina contributes to

• the src repository (c5426ce3a629b003c419fb3d64b29691b061b379);
• the ports repository (cee823d878da8a75c61b01fceba4d8bbcd2a7f5d);
• the doc repository (8a25677c39aa9eda7c3fedc6ad6b4acef061ada7).

She has also contributed to the now archived
https://github.com/freebsd/freebsd-quarterly repository.

Approved by: carlavilla (mentor)
Differential Revision: https://reviews.freebsd.org/D39362