Choose more reasonable default image generation parameters

[markasoftware]

Or, at least, give some good examples of reasonable generation parameters in the readme. The current parameters make a very pretty-looking image, but in fact they aren't very representative of real star tracker photos, which will have few (if any) stars in them. This mainly amounts to turning down the reference brightness, but we should also figure out a more reasonable sensitivity in order to get the shot noise right (Liebe "star tracker accuracy tutorial" paper seems to have some ideas, but could be outdated?)