docs: add MIT license and paper citation

[iamiaud]

Summary

• Add LICENSE (MIT, Copyright (c) 2026 InstaDeep Ltd) — matches the format used in other InstaDeep MIT repos (catx, tunbert).
• Replace the Citation placeholder in README.md with the arXiv BibTeX entry (arXiv:2601.03930).
• Add a License section to the README linking to LICENSE.

Test plan

• Docs-only change. Verified BibTeX renders as a fenced code block and the License link resolves to LICENSE.

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