Add pipeline tag, link to paper

This PR adds the `pipeline_tag` to make sure the model can be found at https://huggingface.co./models?pipeline_tag=text-classification.
It also ensures it is linked to the paper [GuardReasoner: Towards Reasoning-based LLM Safeguards](https://huggingface.co./papers/2501.18492).

README.md

@@ -9,11 +9,9 @@ tags:
 model-index:
 - name: GuardReasoner 1B
   results: []
+pipeline_tag: text-classification
 ---
 
-
 # GuardReasoner 1B
 
-This model is a fine-tuned version of [meta-llama/Llama-3.2-1B](https://huggingface.co/meta-llama/Llama-3.2-1B) via R-SFT and HS-DPO.
-
-
+This model is a fine-tuned version of [meta-llama/Llama-3.2-1B](https://huggingface.co/meta-llama/Llama-3.2-1B) via R-SFT and HS-DPO. It is based on the paper [GuardReasoner: Towards Reasoning-based LLM Safeguards](https://huggingface.co/papers/2501.18492).