Add `library_name: transformers` to model card metadata

This PR improves the model card by adding the `library_name: transformers` tag to the metadata. This will enable the automated "how to use" widget on the Hugging Face Hub, providing users with a quick and easy way to interact with the model using the `transformers` library.

Evidence from `config.json` and `tokenizer_config.json` indicates full compatibility with `transformers`, showcasing `GPT2LMHeadModel` architecture and `GPT2Tokenizer`.

README.md

@@ -1,14 +1,15 @@
 ---
-license: apache-2.0
+base_model:
+- openai-community/gpt2-medium
 datasets:
 - databricks/databricks-dolly-15k
 language:
 - en
+license: apache-2.0
 metrics:
 - rouge
-base_model:
-- openai-community/gpt2-medium
 pipeline_tag: text-generation
+library_name: transformers
 ---
 
 # MiniLLM-gpt2-340M