How can I make an addon that stores arbitrary data with a .blend file? What I'm trying to do

I'm trying to make an addon that can import Minecraft model files. Unlike many 3D files, Minecraft files use a hereditary system where each model file can have a parent file which it inherits elements from. As such, I need a way of locating the parent file referenced in the model, which is represented as it's path relative to a "resource pack root directory"

How I'm going to do it

For each blend file, I'll have the user specify the resource pack root directory somewhere in the properties panel. The problem is that I don't know how to actually store this value once it's set. So how do I store it?
You can set this data inside custom properties: https://docs.blender.org/manual/en/2.79/data_system/custom_properties.html If you do it the python way as described below, it's just adding some property to the object. If it's something blender is not familiar with, the Ui will display it a custom property to you.