Update README.md

Author: PeterMitrano

README.md

@@ -6,6 +6,8 @@
 # Installation
 ```shell
 pip install pytorch-kinematics
+# Alternatively, if you want to load from mujoco's XML format, use:
+pip install pytorch-kinematics[mujoco]
 ```
 
 For development, clone repository somewhere, then `pip3 install -e .` to install in editable mode.