Add MuJoCo Playground to environment repositories list

Author: Mayankm96

README.md

@@ -7,6 +7,7 @@ Environment repositories using the framework:
 
 * **`Isaac Lab`** (built on top of NVIDIA Isaac Sim): https://github.com/isaac-sim/IsaacLab
 * **`Legged-Gym`** (built on top of NVIDIA Isaac Gym): https://leggedrobotics.github.io/legged_gym/
+* **`MuJoCo Playground`** (built on top of MuJoCo MJX and Warp): https://github.com/google-deepmind/mujoco_playground/
 
 The main branch supports **PPO** and **Student-Teacher Distillation** with additional features from our research. These include: