Delete outdated PettingZoo example.

Context: #269
PiperOrigin-RevId: 755801265
Change-Id: I827581a29680a24835679016457905c6e83e072f

Author: jzleibo

README.md

@@ -2,13 +2,18 @@
 
 *A suite of test scenarios for multi-agent reinforcement learning.*
 
+<!-- linter off -->
 <!-- GITHUB -->
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 <!-- /GITHUB -->
+<!-- linter on -->
+
+<!-- disableFinding(SNIPPET_INVALID_LANGUAGE) -->
+<!-- disableFinding(IMAGE_ALT_TEXT_INACCESSIBLE) -->
 
 <div align="center">
   <img src="https://github.com/google-deepmind/meltingpot/blob/main/docs/images/meltingpot_montage.gif?raw=true"
@@ -143,13 +148,12 @@ variants, which you select with the `--level_name` flag.
 
 ### Training agents
 
-We provide two example scripts: one using
-[RLlib](https://github.com/ray-project/ray), and another using
-[PettingZoo](https://github.com/Farama-Foundation/PettingZoo) with
-[Stable-Baselines3](https://github.com/DLR-RM/stable-baselines3) (SB3). Note
-that Melting Pot is agnostic to how you train your agents, and as such, these
-scripts are not meant to be a suggestion on how to achieve good scores in the
-task suite.
+We provide an illustrative example script using
+[RLlib](https://github.com/ray-project/ray). However, note
+that Melting Pot is agnostic to how you train your agents, and this
+script is not meant to be a suggestion for how to achieve a good score
+in the task suite. The authors of the suite never used this example training
+script in their own work.
 
 #### RLlib
 
@@ -170,28 +174,13 @@ cd examples/rllib
 python self_play_train.py
 ```
 
-#### PettingZoo and Stable-Baselines3
-
-This example uses a PettingZoo wrapper with a fully parameter shared PPO agent
-from SB3.
-
-The PettingZoo wrapper can be used separately from SB3 and
-can be found [here](https://github.com/google-deepmind/meltingpot/blob/main/examples/pettingzoo/utils.py).
-
-```shell
-cd <meltingpot_root>
-pip install -r examples/requirements.txt
-cd examples/pettingzoo
-python sb3_train.py
-```
-
 ## Documentation
 
 Full documentation is available [here](https://github.com/google-deepmind/meltingpot/blob/main/docs/index.md).
 
 ## Citing Melting Pot
 
-If you use Melting Pot in your work, please cite the accompanying article:
+If you use Melting Pot in your work, please cite the accompanying articles:
 
 ```bibtex
 @inproceedings{leibo2021meltingpot,
@@ -209,6 +198,15 @@ If you use Melting Pot in your work, please cite the accompanying article:
 }
 ```
 
+```bibtex
+@article{agapiou2022melting,
+  title={Melting Pot 2.0},
+  author={Agapiou, John P and Vezhnevets, Alexander Sasha and Du{\'e}{\~n}ez-Guzm{\'a}n, Edgar A and Matyas, Jayd and Mao, Yiran and Sunehag, Peter and K{\"o}ster, Raphael and Madhushani, Udari and Kopparapu, Kavya and Comanescu, Ramona and Strouse, {DJ} and Johanson, Michael B and Singh, Sukhdeep and Haas, Julia and Mordatch, Igor and Mobbs, Dean and Leibo, Joel Z},
+  journal={arXiv preprint arXiv:2211.13746},
+  year={2022}
+}
+```
+
 ## Disclaimer
 
 This is not an officially supported Google product.