Add pipeline tag

This PR adds the `pipeline_tag` to the model card. This ensures the model can be found at https://huggingface.co/models?pipeline_tag=text-to-video.

README.md

@@ -1,5 +1,6 @@
 ---
 license: mit
+pipeline_tag: text-to-video
 ---
 
 # T2V-Turbo