Add model card

This PR adds a model card for the paper [Unified Vision-Language-Action Model](https://huggingface.co/papers/2506.19850), setting the license to Apache 2.0, adding a robotics pipeline tag and the Transformers library name. It also adds a link to the project page and code repository to facilitate easy navigation.

README.md

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+---
+license: apache-2.0
+pipeline_tag: robotics
+library_name: transformers
+---
+
+This repository contains the model weights for the paper [Unified Vision-Language-Action Model](https://huggingface.co/papers/2506.19850).
+
+Project page: https://robertwyq.github.io/univla.github.io
+
+Code: https://github.com/OS-Copilot/OS-Atlas