Add MIT license

[MaxGhenis]

This repository had no license file, which by default reserves all rights and gives nobody else permission to reuse its contents. This PR adds the MIT License for the code and a README section that puts original text and figures under CC BY 4.0.

This is part of a license pass across the PolicyEngine, Thesis Institute, Axiom Foundation, UBI Center and Cosilico repositories (2026-09-17): MIT for code, CC BY 4.0 for original text and figures, and AGPL-3.0 retained for the model packages that derive from OpenFisca.

A scan of the default branch (dependency manifests at any depth, then Python, notebook and Quarto sources) found no dependency on PolicyEngine's AGPL-licensed Python packages.

Notes:

• This repository's CI requires a changelog fragment in changelog.d/ on every PR, and a fragment that lands on the default branch drives the version bump and package release. This PR leaves the fragment out on purpose, so the changelog check fails and merging stays a maintainer's decision: add a fragment such as changelog.d/license-mit.added.md to ship this as its own release, or fold it into the next one.

Contributors who object to these terms for their contributions can open an issue on this repository.

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Project	Deployment	Actions	Updated
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[vahid-ahmadi]

Reviewed this with a fresh pass, checking the licence-compatibility question independently rather than taking the scan on trust — it is the one thing here with consequences for the five downstream repos.

The core claim holds. MIT is correct. I scanned the whole dependency closure, not just the direct list: everything resolves to BSD / MIT / Apache-2.0 apart from rpy2 (GPL-2.0-or-later), and MPL-2.0 for certifi, tqdm and pathspec, which is file-level copyleft and imposes nothing here. No policyengine-* package appears in the lock file at all, so the AGPL claim is confirmed. pytorch-tabular is MIT and quantile-forest Apache-2.0, so the mdn extra raises nothing.

The LICENSE text is verbatim OSI MIT, and the CC BY 4.0 split in the README is clean — disjoint subject matter, MIT scoped to "the Software", with a sensible third-party carve-out.

To do

Before merging

• Add the licence to packaging metadata. pyproject.toml has no license field, no license-files and no classifiers, so PyPI will keep publishing license: None — which microimpute has no licence, so all rights are reserved #197 says has persisted across all 53 releases. The LICENSE file satisfies JOSS, but not downstream consumers resolving a licence from installed metadata. Add license = "MIT", license-files = ["LICENSE"], and the License :: OSI Approved :: MIT License classifier.
• Sequence against Declare subpackages instead of shipping the source tree as package data #216. Declare subpackages instead of shipping the source tree as package data #216 edits the same [project] block — it adds classifiers, two authors and [project.urls]. GitHub shows no conflict today only because this PR touches just LICENSE and README; the moment it adds classifiers the two collide. Merge Declare subpackages instead of shipping the source tree as package data #216 first, then rebase this and append the licence classifier to the list it introduces.
• Get an explicit ack from @baogorek. microimpute has no licence, so all rights are reserved #197 names the substantial copyright holders; three of the four are PolicyEngine staff, but baogorek is external (9 commits on main). The PR offers opt-out — "contributors who object can open an issue" — and silence is treated as consent. An affirmative sign-off is cheap now and very expensive to retrofit once downstream repos have relied on the grant.

Worth doing in the same PR

• Note the GPL implication of the matching extra in the README. microimpute neither bundles nor links rpy2, the import is guarded, and the default install contains no GPL code — so MIT is right and this is not a violation. But a user who installs microimpute[matching] and redistributes that environment is bound by GPL-2+ for the combined work. One sentence pre-empts a JOSS reviewer asking.
• Add changelog.d/197.added.md and merge normally. The premise in the description does not hold: main has no branch protection (gh api .../branches/main/protection → 404), so the red changelog check is advisory and the merge button stays enabled. has-functional-changes.sh also lists LICENSE* and README.md in IGNORE_DIFF_ON, so this is classified as no functional change. An added fragment gives a minor bump, 3.1.1 → 3.2.0, which is right — and a release is needed anyway to push the metadata above onto PyPI.

Optional

• Copyright year 2025-2026 rather than 2025 — first commit is 2025-02-25 and work continues into 2026. (microdf uses an individual holder, but "PolicyEngine" is the better choice here with four authors and org ownership.)
• A licence line in paper/ — grep -rni "licen\|copyright" paper/ returns nothing, so a reader of the manuscript alone has no signal.
• license: MIT in CITATION.cff once this lands (that file arrives in Add JOSS paper for submission #201).

Not caused by this PR

The red Lint check is pre-existing. I checked the base commit rather than assuming: ruff format --check reports the same five files on main as on this branch, and none of them are touched here. Worth not letting "CI is red anyway" obscure the changelog point above.