Adding `transformers` tag to the library field

This would:
1. Make the model more visible to people searching for `transformers` based models
2. Populate the "Use the model" field in the model card

README.md

@@ -22,6 +22,7 @@ tags:
 language:
 - en
 pipeline_tag: image-text-to-text
+library_name: transformers
 ---
    
 # granite-docling-258m