add protobuf to pip installation instruction

README.md

@@ -486,7 +486,7 @@ Find below some example scripts on how to use the model:
 
 First, install the Python packages that are required:
 
-`pip install transformers accelerate sentencepiece`
+`pip install transformers accelerate sentencepiece protobuf`
 
 ```python
 from transformers import T5ForConditionalGeneration, T5Tokenizer