add library tag

This will enable the use in `transformers` drop-down widget too

README.md

@@ -4,6 +4,7 @@ language:
 - zh
 - en
 pipeline_tag: text-generation
+library_name: transformers
 ---
 <div align="center">
 <img src="https://github.com/OpenBMB/MiniCPM/blob/main/assets/minicpm_logo.png?raw=true" width="500em" ></img>