feat(docs): Add GOG to the README (endless-sky#10437)

Author: TomGoodIdea

README.md

@@ -10,7 +10,7 @@ See the [player's manual](https://github.com/endless-sky/endless-sky/wiki/Player
 
 ## Installing the game
 
-Official releases of Endless Sky are available as direct downloads from [GitHub](https://github.com/endless-sky/endless-sky/releases/latest), on [Steam](https://store.steampowered.com/app/404410/Endless_Sky/), and on [Flathub](https://flathub.org/apps/details/io.github.endless_sky.endless_sky). Other package managers may also include the game, though the specific version provided may not be up-to-date.
+Official releases of Endless Sky are available as direct downloads from [GitHub](https://github.com/endless-sky/endless-sky/releases/latest), on [Steam](https://store.steampowered.com/app/404410/Endless_Sky/), on [GOG](https://gog.com/game/endless_sky), and on [Flathub](https://flathub.org/apps/details/io.github.endless_sky.endless_sky). Other package managers may also include the game, though the specific version provided may not be up-to-date.
 
 ## System Requirements