Add pipeline tag

This PR adds the `pipeline_tag` metadata to the model card, making the model discoverable under the text-to-speech pipeline on the Hugging Face Hub.

README.md

@@ -1,7 +1,8 @@
 ---
 license: apache-2.0
-
+pipeline_tag: text-to-speech
 ---
+
 <div align="center">
     <h1>
     FireRedTTS3