[Add] about sentencepiece

README.md

@@ -21,6 +21,11 @@ widget:
 This repository provides a large sized Japanese GPT-2 model. The model was trained by [ABEJA, Inc](https://abejainc.com/en/)
 
 # How to use
+First, install sentencepiece. We have confirmed behavior with the latest version August 2022. (Skip if not necessary.)
+
+``` shell
+pip install sentencepiece
+```
 
 When using pipeline for text generation.