Add dataset card metadata, paper link and link to code repository

This PR adds metadata (task category, license) to the dataset card, a link to the paper and a link to the code repository to improve discoverability.

README.md

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+---
+task_categories:
+- image-classification
+license: apache-2.0
+---
+
+This repository contains the OpticalRS-13M dataset, as described in the paper [Harnessing Massive Satellite Imagery with Efficient Masked Image Modeling](https://huggingface.co/papers/2406.11933).
+
+Code: [https://huggingface.co/initiacms/SelectiveMAE](https://huggingface.co/initiacms/SelectiveMAE)