Added thumbnail for timestep example

collection/timestep/example.md

@@ -4,6 +4,7 @@ title: Time-step
 brief: This example shows how to speed up or slow down animations in a collection proxy by changing the time step of the collection proxy.
 author: Defold Foundation
 scripts: controller.script, game.script
+thumbnail: thumbnail.png
 ---
 
 The setup consists of a `timestep.collection` and a `game.collection`.