Document using Material category in custom materials

It is not documented anywhere how to allow changing custom material back to standard material. It appears we need to add a category property named "Material" in the metadata to trigger the inclusion of those fields in Inspector.

In short, at least one metadata property needs to have

category="Material" 

defined, otherwise we will not include the Material group in Inspector, which will result in a situation where the material cannot be changed to any other material anymore.