When you buy this game on that other distribution platform from Valve you can download an instruction manual there. Nothing spectecular but a neat little extra, especially since it looks like a used manual of an 80's/90's console game. I think you should add it to the extras.
A few other extras usually found in other games here on GoG like avatars or wallpapers would be nice too.

I second this request. I assume it's something easy to do, and having glanced at the manual online, it looks pretty cool!