Update README.md

The CodeT5+ model is truly impressive! We would like to contribute by updating the README to include information about the base model. This is to help address the missing details in the model card.

README.md

@@ -1,5 +1,7 @@
 ---
 license: bsd-3-clause
+base_model:
+- Salesforce/codegen-350M-multi
 ---
 
 # CodeT5+ 220M