Add pipeline tag and library name

This PR improves the model card by adding
- ensures the model can be found at https://huggingface.co/models?pipeline_tag=text-to-audio&sort=trending
- adds the transformers library to the model card.

README.md

@@ -2,7 +2,10 @@
 language:
 - en
 - zh
+pipeline_tag: text-to-audio
+library_name: transformers
 ---
+
 # SongGeneration
 
 <p align="center">