Add model card

This PR adds a model card for the paper [MuCodec: Ultra Low-Bitrate Music Codec](https://huggingface.co/papers/2409.13216).

It also adds a relevant pipeline tag, ensuring people can find the model at https://huggingface.co/models?pipeline_tag=audio-to-audio, a link to the Github repository, as well as the license.

README.md

@@ -0,0 +1,8 @@
+---
+license: mit
+pipeline_tag: audio-to-audio
+---
+
+This repository contains the model of the paper [MuCodec: Ultra Low-Bitrate Music Codec](https://huggingface.co/papers/2409.13216).
+
+Code: https://github.com/xuyaoxun/MuCodec