docs(readme): link to migration guide (openai#761)

Author: stainless-bot

README.md

@@ -14,6 +14,9 @@ The API documentation can be found [here](https://platform.openai.com/docs).
 
 ## Installation
 
+> [!IMPORTANT]
+> The SDK was rewritten in v1, which was released November 6th 2023. See the [v1 migration guide](https://github.com/openai/openai-python/discussions/742), which includes scripts to automatically update your code.
+
 ```sh
 pip install openai
 ```