Add transformers version constraint to pip install command

Our model does not work with the latest transformers due to an unresolved issue.
Added a note next to the pip install command.

README.md

@@ -62,8 +62,11 @@ Evaluation scores were produced using the python libraries [SacreBLEU](https://g
 
 Ensure you have the prerequisite python libraries installed:
 
-```bsdh
-pip install transformers sentencepiece
+```bash
+# The constraint imposed on the transformers version below
+# is due to the following issue:
+#    https://github.com/huggingface/transformers/issues/26271
+pip install sentencepiece "transformers>4.26.1<=4.30.2"
 ```
 
 ```python