Added skybox thumbnail

model/skybox/example.md

@@ -4,6 +4,7 @@ title: Skybox
 brief: This example shows how to create a skybox using a cubemap texture.
 author: Defold Foundation
 scripts: skybox.fp, skybox.vp
+thumbnail: skybox.png
 ---
 
 This example shows how to create a skybox. A skybox is a technique that makes the scene look bigger and more impressive by wrapping the viewer with a texture that goes around the camera 360 degrees.