Update README.md

Author: simon-lc

README.md

@@ -12,6 +12,9 @@ A differentiable simulator for robotics.
 ### Atlas drop
 <img src="examples/animations/atlas_drop.gif" width="100"/>
 
+### REx Hopper drop
+<img src="examples/animations/rexhopper_square.gif" width="100"/>
+
 ### Astronaut
 <img src="examples/animations/astronaut.gif" width="200"/>