EXT_mesh_gpu_instancing: Suggest extensionRequired usage (#2405)

extensions/2.0/Vendor/EXT_mesh_gpu_instancing/README.md

@@ -57,6 +57,8 @@ All attribute accessors in a given node **must** have the same `count`.
 
 > **Implementation Note:** When instancing is used on the node, the non-instanced version of the mesh should not be rendered.
 
+Because the extension does not provide a way to specify a non-instanced fallback, files that use the extension **should** specify it in `extensionsRequired` array.
+
 ## Transformation Order
 
 When using instancing, the instance transform matrix is constructed by multiplying translation (if present), rotation (if present), and scale in the same order as they are for nodes: