Add model card for UniDisc

This PR adds a model card by linking it to the paper [Unified Multimodal Discrete Diffusion](https://huggingface.co/papers/2503.20853).
It also adds the appropriate pipeline tag and the library name.

README.md

@@ -0,0 +1,13 @@
+---
+pipeline_tag: image-text-to-text
+library_name: transformers
+---
+
+This repository contains the checkpoints for the model presented in the paper [Unified Multimodal Discrete Diffusion](https://huggingface.co/papers/2503.20853).
+
+The code is available at [https://github.com/aswerdlow/unidisc](https://github.com/aswerdlow/unidisc).
+The project page is available at [https://unidisc.github.io/](https://unidisc.github.io/).
+
+The UniDisc checkpoints are available on Hugging Face:
+* [Interleaved Image-Text](https://huggingface.co/aswerdlow/unidisc_interleaved)
+* [Non-Interleaved (Static)](https://huggingface.co/aswerdlow/unidisc_non_interleaved)