Update README.md

The Prometheus 2 model is truly impressive! We would love to contribute by updating the README to include the base_model information. This will help address the current gap in the model card.

README.md

@@ -14,6 +14,8 @@ metrics:
 - spearmanr
 - kendall-tau
 - accuracy
+base_model:
+- mistralai/Mistral-7B-Instruct-v0.2
 ---
 ## Links for Reference