Add CONTRIBUTING.md.

CONTRIBUTING.md

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+# Contributing to Umami
+
+Thanks for your interest in contributing to Umami! This document outlines the process for contributing code.
+
+## Branching
+
+Umami uses the following long-lived branches:
+
+- `master` — stable, released code. **Do not open PRs against `master`.**
+- `dev` — active development. **All pull requests should target `dev`.**
+
+Feature branches and fixes are merged into `dev`, and `dev` is periodically merged into `master` for releases.
+
+## Submitting a Pull Request
+
+1. Fork the repository and create your branch from `dev`:
+   ```bash
+   git checkout dev
+   git pull origin dev
+   git checkout -b my-feature
+   ```
+2. Make your changes. Keep PRs focused — one logical change per PR.
+3. Ensure the project builds and lints cleanly:
+   ```bash
+   pnpm install
+   pnpm build
+   pnpm lint
+   ```
+4. Push your branch and open a pull request **against the `dev` branch**.
+5. Fill in the PR description with what changed and why. Link any related issues.
+
+PRs opened against `master` will be asked to retarget `dev`.
+
+## Reporting Issues
+
+- Search [existing issues](https://github.com/umami-software/umami/issues) before opening a new one.
+- For bugs, include reproduction steps, expected vs. actual behavior, and your environment (Umami version, database, browser).
+- For feature requests, describe the use case before the proposed solution.
+
+## Development Setup
+
+See the [README](./README.md) for instructions on installing dependencies, configuring the database, and running Umami locally.
+
+## License
+
+By contributing, you agree that your contributions will be licensed under the [MIT License](./LICENSE).