update readme

Author: nch0w

README.md

@@ -31,7 +31,7 @@ uv sync
 
 ### Set up environment variables
 
-Set the `OPENAI_API_KEY` environment variable to your OpenAI API key. Executing the test script below will send a few queries to `gpt-4.1-mini`.
+Set the `OPENAI_API_KEY` environment variable to your OpenAI API key. Executing the test script below will send a few queries to `gpt-4.1-mini`, which we use as our default judge model.
 
 ### Run a test script demonstrating the PRBO reward function