Add pipeline tag

This PR ensures the model can be found at https://huggingface.co/models?pipeline_tag=text-to-video&sort=trending.

README.md

@@ -3,6 +3,7 @@ license: apache-2.0
 language:
 - en
 library_name: diffusers
+pipeline_tag: text-to-video
 ---
 <p align="center">
 <img src="https://huggingface.co/rhymes-ai/Allegro/resolve/main/banner_white.gif">