Add pipeline tag

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

README.md

@@ -241,6 +241,7 @@ extra_gated_description: Mistral AI processes your personal data below to provid
   policy</a>.
 extra_gated_button_content: Submit
 library_name: vllm
+pipeline_tag: image-text-to-text
 ---
 
 # Model Card for Pixtral-Large-Instruct-2411