Add Cube requirements for ItemArmorGeoLayer to Wiki

[Dariensg]

Currently, the wiki doesn't give any specifics for how the ItemArmorGeoLayer needs to be set up in the model of the entity it is being added to. The assumptions for this render layer require the armor bones to have a single cube the shape and size of where you want that armor piece to be. If no cubes are included, the layer will crash. If multiple are added, only the first will be used.

Since this is the standard usage of a library-supplied render layer, details for usage should be easily documented.