Add pipeline tag

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

README.md

@@ -1,5 +1,6 @@
 ---
 license: bsd-3-clause
+pipeline_tag: text-to-image
 datasets:
 - ILSVRC/imagenet-1k
 tags: