There are instructions in the folder that you can open from the Mod app in the game
Modders have to write story as .ink files (easy to do) and dev added some more commands to write in .ink files and to use images you have to put the images in some specific subfolders. Pretty easy
Anyway, I gave a look at how it works and I saw there's only boolean logic, I hope in future he'll add a feature to let modder add some different type of variables to have the possibility to add counters or levels of something! In this way, there is a good base for story telling, but adding different types could help to make deeper stories (like the girl who betray only if you had a certain amount of bad chooses, not depending in which ones)