Update README.md with Atropos (#458)

Author: zafstojano

README.md

@@ -103,6 +103,7 @@ If you have ideas for dataset generators please create an issue here or contact
 Following is a list of awesome projects building on top of Reasoning Gym:
 - [Verifiers: Reinforcement Learning with LLMs in Verifiable Environments](https://github.com/willccbb/verifiers)
 - [(NVIDIA) ProRL: Prolonged Reinforcement Learning Expands Reasoning Boundaries in Large Language Models](https://arxiv.org/abs/2505.24864)
+- [Atropos - Nous Research's LLM RL Gym](https://github.com/NousResearch/atropos)
 
 ## 📝 Citation