OMI_link should add note for implementations without physics engine

[lyuma]

OMI_link and OMI_seat currently depends (optionally?) on OMI_physics_body (and by extension, OMI_physics_shape).

I was worried about the dependency on OMI_physics_body reducing adoption of OMI_link which is otherwise a simple spec.

I want to add a note to OMI_link that it is acceptable to implement this and read the shape information from the physics properties, without necessitating a whole physics engine implementation.