✨ add LICENSE

LICENSE.md

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+This repository is licensed according to Ecma International TC39's [Intellectual Property Policy](https://github.com/tc39/how-we-work/blob/HEAD/ip.md). In particular:
+
+- Natural language text is licensed under the [Alternative copyright notice of the Ecma text copyright policy](https://ecma-international.org/policies/by-ipr/ecma-text-copyright-policy/#alternative-copyright-notice).
+- Source code is licensed under Ecma's MIT-style [Ecma International Policy on Submission, Inclusion and Licensing of Software](https://www.ecma-international.org/memento/Policies/Ecma_Policy_on_Submission_Inclusion_and_Licensing_of_Software.htm).
+- Contributions are only accepted from either representatives of Ecma members or signatories of TC39's [Contributor Form](https://tc39.github.io/agreements/contributor/).