adding demo script branch

python/mujoco/usd/demo.py

@@ -0,0 +1,19 @@
+import mujoco
+from mujoco.usd import exporter
+
+if __name__ == "__main__":
+
+  # load a model to mujoco
+  m = mujoco.MjModel.from_xml_path("/Users/abhishek/Documents/research/mujoco/model/cards/cards.xml")
+  d = mujoco.MjData(m)
+
+  # create an instance of the USDExporter
+  exp = exporter.USDExporter(model=m)
+
+  mujoco.mj_step(m, d)
+
+  exp.update_scene(d)
+
+  exp.save_scene(filetype="usd")
+
+