Add pipeline tag

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

README.md

@@ -3,6 +3,7 @@ license: apache-2.0
 language:
 - en
 library_name: diffusers
+pipeline_tag: text-to-image
 ---
 <div align="center">
 <h1>StoryMaker: Towards consistent characters in text-to-image generation</h1>