I made an example mod which you can download and edit here: https://cdn.discordapp.com/attachments/373987568779395072/400002960366764033/cry... . Simply create a new folder called mods in the data folder, and extract that zip file there (so that "data/mods/crystal_sword/mod.xml" exists). Then start the game or hit Ctrl+Shift+M to reload mods live (just don't reload mods during events, as it will cancel events). Any folder in the mods folder that has a mod.xml will be loaded automatically for now, I'll have mod management in game later.
You can also look at the data/core folder, as that is actually structured exactly like a mod.
And feel free to drop by #modding in our discord, as there's lots of discussions about modding going on there!