Add upstream attribution notice

NOTICE.md

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+# Notices
+
+## This repository
+
+Core ML conversion of the laya-multilingual decision model, by Fluid Inference.
+Licensed under the Apache License, Version 2.0; see `LICENSE`.
+
+## Upstream model and code
+
+The converted weights and the reference implementation are the work of
+Convai Innovations and contributors, used under the Apache License, Version 2.0.
+
+- Weights: [`convaiinnovations/laya`](https://huggingface.co/convaiinnovations/laya),
+  subfolder `multilingual`, revision `1c5edc17a7acd8701df6fc341c0d179f1c62c982`.
+- Reference implementation: [`NandhaKishorM/laya`](https://github.com/NandhaKishorM/laya).
+
+The encoder is [`jhu-clsp/mmBERT-base`](https://huggingface.co/jhu-clsp/mmBERT-base),
+and `tokenizer.json` carries its vocabulary, redistributed under that model's terms.
+
+No weights were retrained. The conversion changes tensor precision and fixes input
+shapes; the parameters are the upstream ones. This is an independent conversion and
+is not an official Convai Innovations release.