[README] Add Oumi to "Supported RL Tools" also

I see there is a second section in the README for tool integrations. Could we please add Oumi to this as well?

This relates to meta-pytorch#195 that added Oumi to the very top of the README

Author: stefanwebb

README.md

@@ -263,6 +263,9 @@ See the [SkyRL example](https://skyrl.readthedocs.io/en/latest/examples/openenv.
 ### ART
 See the [ART example](https://art.openpipe.ai/integrations/openenv-integration) on how OpenEnv environments can be used to train models with ART.
 
+### Oumi
+See the [Oumi example](https://github.com/oumi-ai/oumi/blob/main/notebooks/Oumi%20-%20OpenEnv%20GRPO%20with%20trl.ipynb) on how OpenEnv environments can be used to train models with Oumi.
+
 ## Example Environments
 
 ### Echo Environment