Add license badge to README.

PiperOrigin-RevId: 460437521
Change-Id: I25514343a6b0f507c661261a20d1ad07465491fc

README.md

@@ -4,6 +4,9 @@
   <a href="https://github.com/deepmind/mujoco/actions/workflows/build.yml?query=branch%3Amain" alt="GitHub Actions">
     <img src="https://img.shields.io/github/workflow/status/deepmind/mujoco/build/main">
   </a>
+  <a href="https://github.com/deepmind/mujoco/blob/main/LICENSE" alt="License">
+    <img src="https://img.shields.io/github/license/deepmind/mujoco">
+  </a>
 </p>
 
 **MuJoCo** stands for **Mu**lti-**Jo**int dynamics with **Co**ntact. It is a
@@ -194,4 +197,4 @@ This is not an officially supported Google product.
 [documentation source files]: https://github.com/deepmind/mujoco/tree/main/doc
 [mujoco.readthedocs.io]: https://mujoco.readthedocs.io
 [Python Bindings]: https://mujoco.readthedocs.io/en/latest/python.html#python-bindings
-[PyPI]: https://pypi.org/project/mujoco/
+[PyPI]: https://pypi.org/project/mujoco/