Will you add achievements for each of the endings in Project Kat on Steam?
Probably not. Our achievements aren't exhaustive because we consider certain endings to be secrets. Having achievements would give those away and spoil the fun of finding them. This doesn't just apply to endings - we consider the achievements to simply be a guide for some of the other things you can try in the game.
IIRC, I've played some games on Steam that actually had secret achievements. They usually have different name/description saying its a secret in the game's achievements list. After you obtain/unlock the secret achievement in-game, the achievement's name/description changes to reveal its true name/description.