Add link to paper, github repo, visual grounding task category

This PR adds a link to the paper and github repo, as well as the visual grounding task category.

README.md

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+---
+task_categories:
+- visual-grounding
+license: unknown
+---
+
+This repository contains the dataset used in the paper [VLM-R1: A Stable and Generalizable R1-style Large Vision-Language Model](https://huggingface.co/papers/2504.07615).
+
+Code: https://github.com/om-ai-lab/VLM-R1