Add pipeline tag

This PR ensures the model has the same tag as llava, qwen2-vl, pixtral, etc: https://huggingface.co/models?pipeline_tag=image-text-to-text&sort=trending

README.md

@@ -1,5 +1,6 @@
 ---
 license: llama2
+pipeline_tag: image-text-to-text
 ---
 
 # UGround