apps/scripts/satnogs_waterfall.gp: Add usage documentation

Since this waterfall sink is useful by itself (without satnogs), I added these short comments (not sure whether this place is appropriate).

Source: https://github.com/satnogs/satnogs-client/blob/68cbdb0812c63fa374ec5013320110126a3d96d5/satnogsclient/observer/observer.py