Development currently on hold due to other priorities.
Add this submodule with the following git command (Assuming the root of your Git project is the Unity project folder)
git submodule add https://github.com/iffn/iffnsTreesForUnity.git Assets/iffnsStuff/iffnsTreesForUnity
If you have manually added it, use this one first. (I recommend to close the Unity project first)
git rm Assets/iffnsStuff/iffnsTreesForUnity -r