Update README.md

Hi there! First of all, I want to say that your model is fantastic. We would love to contribute by updating the README to include the base_model information. This is to help address the missing details in the model card. Thank you!

README.md

@@ -1,5 +1,7 @@
 ---
 license: bsd-3-clause
+base_model:
+- Salesforce/codegen-350M-multi
 ---
 # CodeGen (CodeGen-Mono 350M)