Add pipeline tag

This PR ensures the model can be found at https://huggingface.co/models?pipeline_tag=zero-shot-image-classification.

Feel free to update all other models :)

README.md

@@ -1,5 +1,6 @@
 ---
 license: apache-2.0
+pipeline_tag: zero-shot-image-classification
 ---
 <div align="center">