docs: add missing LICENSE and clarify README license section

[mathias3]

Summary

This PR adds explicit licensing metadata to make downstream reuse and attribution clearer.

Changes

• Add missing file (MIT) at repository root\n- Update README section to explicitly reference MIT +
• Keep citation expectation explicit for downstream users of code/data

Why

The README currently says "See LICENSE file for details", but no file is present. Adding it removes ambiguity for open-source downstream projects using this dataset and analysis pipeline.

Notes

• Chosen license text is standard MIT and matches METR's broader public repo licensing pattern.
• If METR prefers a different license for this specific repository, I'm happy to update this PR accordingly.

looks like Add license#28 PR stalled discussion is not available