Improve model card: Add pipeline, library, license, and relevant tags

This PR improves the model card by:
- Populating the metadata YAML block at the top of the README.md.
- Adding the `pipeline_tag: image-to-image` for better discoverability on the Hugging Face Hub, as this model focuses on visual tokenization and reconstruction.
- Specifying `library_name: diffusers` and `license: mit`, which are common for visual models of this type.
- Including additional `tags` such as `visual-tokenization` and `image-reconstruction` to further enhance discoverability and categorization.

The existing arXiv and GitHub links in the model card content have been preserved as they are already correct and the task instructions advise against replacing existing arXiv links if present.

README.md

@@ -1,3 +1,12 @@
+---
+pipeline_tag: image-to-image
+library_name: diffusers
+license: mit
+tags:
+- visual-tokenization
+- image-reconstruction
+---
+
 <div align="center">
 <h1>🚀 WeTok: Powerful Discrete Tokenization for High-Fidelity Visual Reconstruction</h1>