docs: add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to Neural-LAM
+
+Thank you for your interest in contributing to Neural-LAM!
+We welcome bug reports, bug fixes, documentation improvements, and new features.
+
+## Getting started
+
+1. **Open or find an issue.** Before writing code, open a
+   [GitHub issue](https://github.com/mllam/neural-lam/issues) describing what
+   you plan to do. If an issue already exists, comment so others know you are
+   working on it.
+
+2. **Read the existing discussion.** Review the full issue thread and any
+   linked PRs — someone may have already proposed a solution or identified a
+   blocker.
+
+3. **Set up your environment.** Fork the repo via the GitHub UI, then follow
+   the [Installing Neural-LAM section of the README](README.md#installing-neural-lam)
+   using the `--group dev` flag. Then activate the hooks:
+
+   ```bash
+   pre-commit install
+   ```
+
+## Code standards
+
+Code quality is enforced automatically by
+[pre-commit hooks](.pre-commit-config.yaml) (Black, isort, Flake8, mypy,
+Codespell and others). In addition:
+
+- Add **NumPy-style docstrings** and **type annotations** to every public
+  function and class.
+- Keep new code consistent with the patterns already in the codebase.
+
+## Before you push
+
+Run **both** checks locally — they are the same ones CI will run:
+
+```bash
+pre-commit run --all-files
+pytest -vv -s --doctest-modules
+```
+
+> **Note:** The first test run downloads ~50 MB of example data via
+> [pooch](https://www.fatiando.org/pooch/).
+
+## Pull requests
+
+1. Push your branch and open a PR against `main`.
+2. Fill in the
+   [pull request template](.github/pull_request_template.md) — it contains the
+   full checklists for authors, reviewers, and assignees, including the
+   CHANGELOG entry format.
+3. A maintainer will review your PR. Small, focused PRs are reviewed faster.
+
+## Getting help
+
+- Join the [mllam Slack workspace]( https://join.slack.com/t/ml-lam/shared_invite/zt-3jyw20n4g-ESRxMPPSijiZ2ZA6Nh8XhA)
+- [Open a GitHub issue](https://github.com/mllam/neural-lam/issues)