Add pipeline tag

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

README.md

@@ -5,6 +5,7 @@ datasets:
 base_model:
 - stable-diffusion-v1-5/stable-diffusion-v1-5
 library_name: diffusers
+pipeline_tag: text-to-image
 tags:
 - text-generation-inference
 ---