But, it is not explained where in the directory this file should be placed
Anywhere you want, the only important thing is that it's inside the textures folder of your namespace.
Check the example packs:
https://www.spigotmc.org/resources/addon-more-itemsadder-minerals-and-custom-armors-for-itemsadder.97688/
https://www.spigotmc.org/resources/armors-custom-cosmetic-armors-for-itemsadder.97687/
