Add initial dataset card

This PR adds an initial dataset card, links to the paper and sets the task category to image-text-to-text

README.md

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+---
+task_categories:
+- image-text-to-text
+---
+
+MedHallTune: A Benchmark and Instruction Tuning Dataset for Mitigating Medical Hallucination in Vision-Language Models
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+This dataset is described in the paper: https://arxiv.org/abs/2502.20780