Update README.md

The Flan-UL2 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

@@ -59,6 +59,8 @@ datasets:
   - qed
   - c4
 license: apache-2.0
+base_model:
+- google/ul2
 ---