Clarify in README.md that we switched to gymnasium. (#824)

Author: ernestum

README.md

@@ -29,9 +29,11 @@ You can read the latest benchmark results [here](https://imitation.readthedocs.i
 ### Prerequisites
 
 - Python 3.8+
-- (Optional) OpenGL (to render Gym environments)
+- (Optional) OpenGL (to render Gymnasium environments)
 - (Optional) FFmpeg (to encode videos of renders)
 
+> Note: `imitation` is only compatible with newer [gymnasium](https://gymnasium.farama.org/) environment API and does not support the older `gym` API.
+
 ### Installing PyPI release
 
 Installing the PyPI release is the standard way to use `imitation`, and the recommended way for most users.