diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
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+# Contributing to ewstools
+
+Thanks for considering a contribution. Bug reports and suggestions go on the
+[issue tracker](https://github.com/ThomasMBury/ewstools/issues); code comes as a pull request
+against `main`.
+
+**Small fixes need none of what follows.** A typo, a broken link, a clearer error message, a bug
+with a test that reproduces it — open the pull request. The rest of this page is for new features.
+
+## Scope
+
+**In scope:** code that computes, tests, or visualises an early warning signal (EWS) indicator on
+the package's data structures (`TimeSeries`, `MultiTimeSeries`), and simulated models that generate
+test data for those indicators. The underlying statistic does not have to be novel or specific to
+this field — variance, autocorrelation, entropy and DFA are all general statistics, and they are in
+scope because the package computes them over a rolling window *as resilience indicators*.
+
+**Out of scope:** general statistical machinery that is not itself an indicator — multiple-testing
+corrections, methods for combining significance across tests, generic fitting or smoothing
+utilities. These belong in SciPy, statsmodels, or a geostatistics library such as PySAL (`esda`);
+where an implementation already exists there, calling it is preferred to reimplementing it.
+
+If you are unsure which side a contribution falls on, that is what the issue below is for.
+
+## Before writing anything larger than a bug fix
+
+Open an issue naming the indicator, its reference, and where in the package it would live, and wait
+for a reply before building. This costs ten minutes and can save a week.
+
+This is a small package maintained alongside a full-time academic job, so a reply can take a couple
+of weeks. **If two weeks pass, bump the issue; if another two pass, open a draft pull request
+referencing it** and the discussion can continue there. Silence means the maintainer is busy, not
+that the answer is no.
+
+## Shape
+
+`helpers.dfa` + `TimeSeries.compute_dfa` + `tests/test_dfa.py` are the pattern to copy — one pure
+function, one thin method, one test with a known answer.
+
+- **New indicator maths goes in a pure function over NumPy arrays** in `ewstools/helpers.py`
+  (`f(values, ...) -> float` or an array), with no dependence on the package's classes. That
+  function is what gets tested and documented.
+- **The class method orchestrates** — it crops to the transition, walks the rolling window, calls
+  the function, and stores the result in `self.ews`. It holds no mathematics of its own.
+- Several older methods in `core.py` compute inline instead. They stay as they are; this rule is
+  for new indicators.
+- Every new public function or method is a commitment. Prefer extending an existing entry point
+  over adding a new one.
+
+## Setting up
+
+```
+git clone https://github.com/<you>/ewstools.git && cd ewstools
+python -m venv .venv && source .venv/bin/activate
+pip install -e . -r requirements_dev.txt
+pytest tests/
+```
+
+One test skips unless TensorFlow is installed, which is expected — the deep-learning classifier is
+optional. The package supports Python 3.9 upwards, so please avoid syntax newer than that.
+
+## Before opening a pull request
+
+- Tests in `tests/` for every new function, including at least one case with a **known answer** — a
+  hand-computed example, a limiting case with an analytic value, or a comparison against an
+  established implementation. `tests/test_dfa.py` shows the idea.
+- A docstring in the NumPy style used in `core.py`, stating the reference for the method.
+  (`helpers.py` is not consistent about this; follow `core.py`.)
+- A **new module** must be exported from `ewstools/__init__.py` and given a section in
+  `docs/source/ewstools.rst`, or it will not appear on Read the Docs. Nothing warns you if you
+  forget — the docs build does not notice an unreferenced module. A new *function* in an existing
+  module needs neither, since the docs use `automodule` with `:members:`.
+- A line in `CHANGELOG.md` under an `## [Unreleased]` heading; add that heading if it is not there,
+  which it usually will not be.
+- `pytest tests/` passing on a branch rebased on current `main`.
+
+The docs build runs in CI with warnings treated as errors, so a malformed docstring will fail the
+pull request.
+
+Merging into `main` does not publish a release; releases are cut separately by tagging a version.
diff --git a/README.md b/README.md
index 9bdd778..b1c0010 100644
--- a/README.md
+++ b/README.md
@@ -137,7 +137,7 @@ Full documentation available on [ReadTheDocs](https://ewstools.readthedocs.io/en
 
 Found a bug or have a suggestion? Please post it on the [issue tracker](https://github.com/ThomasMBury/ewstools/issues).
 
-Contributions are welcome! Feel free to reach out or submit a pull request.
+Contributions are welcome! Please read [CONTRIBUTING.md](CONTRIBUTING.md) first — it sets out what is in scope and how to shape a contribution.
 
 ## Acknowledgements