Update Loading-3D-models.html

To add another troubleshooting step that could be useful. The problem I had in order to make the model visible was that it was hidden in the dark. Once I added and positioned a light source, the model appeared. Maybe someone else could have the same problem and could solve it more quickly than me.

docs/manual/en/introduction/Loading-3D-models.html

@@ -149,6 +149,9 @@
 			scaled differently, and large models may not appear if the camera is
 			inside the model.
 		</li>
+		<li>
+			Try to add and position a light source. The model may be hidden in the dark.
+		</li>
 		<li>
 			Look for failed texture requests in the network tab, like
 			<em>C:\\Path\To\Model\texture.jpg</em>. Use paths relative to your