Add UniCAD dataset license and attribution

LICENSE.md

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+# UniCAD Synthetic Multi-View CAD Images: license and attribution
+
+The UniCAD-generated image files and UniCAD-authored metadata in this repository
+are made available under the **Creative Commons Attribution-NonCommercial-
+ShareAlike 4.0 International License (CC BY-NC-SA 4.0)**:
+
+<https://creativecommons.org/licenses/by-nc-sa/4.0/legalcode>
+
+When using this dataset, please attribute the **UniCAD team** and link to this
+dataset repository. Adaptations must be shared under the same license and may
+not be used for commercial purposes without separate permission from the
+relevant rightsholders.
+
+This license statement applies only to material that the UniCAD contributors
+are authorized to license. It does not relicense third-party material. This
+repository does not redistribute the original Text2CAD descriptions, Text2CAD
+CadQuery programs, or original DeepCAD renders. Users who combine this release
+with upstream resources are responsible for complying with the corresponding
+upstream licenses and terms.
+
+Upstream attribution:
+
+- DeepCAD: <https://github.com/rundiwu/DeepCAD>
+- Text2CAD: <https://github.com/SadilKhan/Text2CAD>
+- Qwen-Image-Edit: <https://huggingface.co/Qwen/Qwen-Image-Edit>
+