Add a link to muax.

Fixes #46.

PiperOrigin-RevId: 521543131

Author: fidlej

README.md

@@ -119,6 +119,8 @@ The following projects demonstrate the Mctx usage:
   AlphaZero on random mazes.
 - [a0-jax](https://github.com/NTT123/a0-jax) — AlphaZero on Connect Four,
   Gomoku, and Go.
+- [muax](https://github.com/bwfbowen/muax) — MuZero on gym-style environments
+(CartPole, LunarLander).
 
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