docs(contributing): add authorship & acknowledgments policy

Every merged PR is automatically credited in CONTRIBUTORS.md and the
JOSS paper's Acknowledgments. Paper authorship is separate and
extended by invitation against four objective criteria (new estimator
family, reference-parity ownership, sustained core contribution, or
paper writing), aligned with ICMJE / JOSS norms.

Adds CONTRIBUTORS.md seed file and a prominent callout in the "Types
of Contributions" block so new contributors see the policy before
opening a PR.

Author: brycewang-stanford

CONTRIBUTING.md

@@ -12,6 +12,11 @@ We welcome contributions to StatsPAI! This document provides guidelines for cont
 4. **Documentation**: Improve docs, examples, or tutorials
 5. **Testing**: Add test cases or improve test coverage
 
+> **On credit**: every merged PR is credited in `CONTRIBUTORS.md` and
+> the paper's Acknowledgments. Paper **authorship** is separate and
+> extended by invitation against objective criteria — see
+> [Authorship & Acknowledgments](#-authorship--acknowledgments) below.
+
 ### Getting Started
 
 1. **Fork the Repository**
@@ -190,6 +195,76 @@ src/statspai/
        })
    ```
 
+## 🧾 Authorship & Acknowledgments
+
+StatsPAI is an academic project that publishes a JOSS paper and
+follow-on methodological work. To keep expectations transparent, we
+separate **acknowledgment** from **paper authorship**.
+
+### Acknowledgments (automatic)
+
+Everyone whose pull request is merged into `main` is automatically
+credited. Specifically:
+
+- Added to `CONTRIBUTORS.md` (name + GitHub handle + area of contribution).
+- Listed in the **Acknowledgments** section of the JOSS paper and any
+  subsequent methodological papers that build on the codebase they
+  touched.
+- Credited in release notes (`CHANGELOG.md`) for the version that
+  ships their change.
+
+No invitation is needed. This is the default and applies to all merged
+contributions — bug fixes, docs, tests, refactors, new estimators.
+
+### Paper Authorship (by invitation, criteria-based)
+
+**By default, code contributors are not co-authors of StatsPAI
+papers.** The core development team extends authorship invitations to
+contributors who meet **at least one** of the following objective
+criteria, consistent with
+[ICMJE](https://www.icmje.org/recommendations/browse/roles-and-responsibilities/defining-the-role-of-authors-and-contributors.html)
+and [JOSS](https://joss.readthedocs.io/en/latest/submitting.html#authorship)
+authorship norms:
+
+1. **New estimator family** — independently designed and implemented a
+   first-class public function family exposed through `sp.*`
+   (e.g., a new `sp.xxx` or a new branch of an existing dispatcher
+   such as `sp.synth(method=...)` / `sp.decompose(method=...)` /
+   `sp.dml(model=...)`), including reference-parity tests.
+2. **Reference alignment ownership** — led the numerical alignment of
+   a non-trivial module against Stata / R / published paper numbers
+   (added tests under `tests/reference_parity/` or
+   `tests/external_parity/` that exercise real external output).
+3. **Sustained core contribution** — ≥ 10 merged PRs **and** ≥ 2,000
+   net lines of production code (excluding generated files, vendored
+   assets, and pure formatting) in core estimator modules (any module
+   listed under "因果 / 处理效应", "面板 / 结构", or "因果发现 / ML" in
+   `CLAUDE.md`).
+4. **Paper writing** — drafted or substantially revised sections of
+   the manuscript, participated in response-to-reviewers, and approved
+   the final submitted version.
+
+Meeting a criterion makes a contributor **eligible** for an
+invitation; the core team issues the invitation in writing and the
+contributor must accept explicitly. Invited authors are expected to
+also satisfy standard ICMJE responsibilities: review and approve the
+final manuscript, and be accountable for the accuracy of the portions
+they contributed.
+
+### What this means in practice
+
+- A one-off bug fix or docs PR → Acknowledgments + `CONTRIBUTORS.md`.
+- Implementing a new estimator with parity tests → eligible for
+  authorship invitation under criterion (1) or (2).
+- Large-scale code contribution across many PRs → eligible under
+  criterion (3).
+- Contributions to paper text or reviewer response → eligible under
+  criterion (4).
+
+If you are actively working toward authorship eligibility, feel free
+to open a discussion or email <brycew6m@stanford.edu> ahead of time so
+scope and criteria can be agreed upon before the work is done.
+
 ## 📞 Getting Help
 
 - **GitHub Issues**: For bugs and feature requests

CONTRIBUTORS.md

@@ -0,0 +1,22 @@
+# Contributors
+
+StatsPAI is built by the people listed here. Every merged pull request
+earns a line in this file and a slot in the Acknowledgments section of
+the project's JOSS paper.
+
+> Paper **authorship** is separate and extended by invitation against
+> objective criteria — see the "Authorship & Acknowledgments" section
+> of [`CONTRIBUTING.md`](CONTRIBUTING.md#-authorship--acknowledgments).
+
+## Core Maintainers
+
+- **Biaoyue (Bryce) Wang** ([@brycewang-stanford](https://github.com/brycewang-stanford)) — project lead, architecture, most estimator modules.
+
+## Contributors
+
+<!--
+Format: - **Name** ([@handle](https://github.com/handle)) — area of contribution (e.g., "did module", "docs", "reference parity for sp.rd").
+New entries go in alphabetical order by handle. Added automatically on PR merge.
+-->
+
+_Be the first! Open a PR and your name lands here._