Add library_name metadata

This PR adds the `library_name` tag to the model card, setting it to `transformers`. This addition correctly reflects the library used for loading and interacting with the model, as evidenced by the `AutoTokenizer` and `AutoModel` imports in the usage example.

Adding this metadata improves the model's discoverability on the Hugging Face Hub and enables proper integration with `transformers` library features, such as the "how to use" button and linking to documentation.

README.md

@@ -1,6 +1,7 @@
 ---
 license: apache-2.0
 pipeline_tag: text-to-image
+library_name: transformers
 ---
 
 ## NextStep-1: Toward Autoregressive Image Generation with Continuous Tokens at Scale