Add a note to README.md

[genZrizzCode]

Add this note to README.md

Reasoning & Context

It just might be me being stupid, but I always forget to give all of my object materials, which causes it to export a useless .3MF file that doesn't have any geometry data.

If you do export it with this flaw, you will get this error:

Python: Traceback (most recent call last):
  File "/Users/(you)/Library/Application Support/Blender/3.5/scripts/addons/io_mesh_3mf/export_3mf.py", line 115, in execute
    self.material_name_to_index = self.write_materials(resources_element, blender_objects)
  File "/Users/(you)/Library/Application Support/Blender/3.5/scripts/addons/io_mesh_3mf/export_3mf.py", line 220, in write_materials
    material_name = material.name
AttributeError: 'NoneType' object has no attribute 'name'

It will export, but only a couple hundred bytes will be in there.

So, all in all, please add this because it will be helpful to new Blender users looking to 3D print their 3D designs.

[genZrizzCode]

@Ghostkeeper