Steam - how to make additional content for players depending on achievements?

This is a question for Steam publishers. I'd like to provide more content for free - for example a comic expanding the lore of the game - for the players who found a secret place in the game. How can it be done?

When the achievement is unlocked, then a free DLC should be proposed / unlocked / shown to the Player. Has anyone did it on Steam? How it should work?

I mean you don't have to necessarily tie it directly to the steam achivement? When you unlock the achivement, you also store anywhere (localstorage) that "player has X unlocked" and work from there.