Adding `transformers` as the library tag

This will help with more visibility, correct download metrics and most importantly the "Use this model" tab will now be populated with the code snippets to run inference on the model.

README.md

@@ -11,6 +11,7 @@ license: apache-2.0
 language:
 - en
 pipeline_tag: text-generation
+library_name: transformers
 ---
 <div align="center">
   <h1>Palmyra-mini</h1>