You compile this if you're interested and throw it next to your StardewValley.exe and run it. It should open the game and a beautiful black box that look atrocious honestly. That's the modding api. You can make a project and reference that to add functionality. Below is my test mod class. Mods go in C:\Users\<USERNAME>\AppData\Roaming\StardewValley\Mods or something like that.
It is currently 7AM EST and I have not slept in over 40 hours, so good night/morning/etc to anyone reading, I'll make this more proper some other time.