Update contributing guide

Author: jeromegamez

.github/CONTRIBUTING.md

@@ -1,12 +1,89 @@
-Thank you for considering to contribute to this project.
+# Contributing
 
-Please don't introduce changes without talking about them to the 
-repository maintainers first - this way we can avoid that you 
-put your valuable time and effort into a Pull Request that 
-eventually could not be merged.
+## Before you start
 
-If you want to provide a bugfix, please include tests to 
-prove that the bug exists and that it is fixed.
+Please do not introduce changes without talking to the repository maintainers first. That way nobody spends time on a pull request that cannot be merged.
 
-Support the project's development and keep it sustainable by becoming a
-[GitHub Sponsor](https://github.com/sponsors/jeromegamez).
+If you want to provide a bug fix, include tests that prove the bug exists and that it is fixed.
+
+## Local setup
+
+1. Install project dependencies:
+
+   ```bash
+   composer setup
+   ```
+
+2. Create local test env config from [tests/.env.dist](../tests/.env.dist):
+
+   ```bash
+   cp tests/.env.dist tests/.env
+   ```
+
+3. Fill in the required values in `tests/.env`:
+
+   - `GOOGLE_APPLICATION_CREDENTIALS`
+   - `TEST_FIREBASE_APP_ID`
+   - `TEST_FIREBASE_RTDB_URI`
+   - `TEST_FIREBASE_TENANT_ID`
+   - `TEST_REGISTRATION_TOKENS`
+   - `TEST_FIRESTORE_CUSTOM_DB_NAME` if you use a custom Firestore database
+
+4. If you run emulator tests, install the Firebase CLI and make sure it can start Auth and Realtime Database emulators. The test suite uses ports `9099` and `9100`.
+
+   ```bash
+   composer test:emulator
+   ```
+
+## Useful commands
+
+### Lint
+
+```bash
+composer lint
+```
+
+Runs Rector, PHP-CS-Fixer, and composer normalization checks.
+
+### Typecheck / static analysis
+
+```bash
+composer analyze
+```
+
+Runs PHPStan.
+
+### Tests
+
+```bash
+composer test:unit
+composer test:integration
+composer test:emulator
+composer test:all
+```
+
+Before pushing, run:
+
+```bash
+composer pre-push
+```
+
+This runs lint fixes, full tests, and backward-compat checks.
+
+### Coverage
+
+```bash
+composer test:coverage
+```
+
+## Notes
+
+- `tests/bootstrap.php` loads `tests/.env` automatically.
+- `tests/bin/reset-project` also reads `tests/.env` and requires `GOOGLE_APPLICATION_CREDENTIALS`.
+  Use only with test or integration projects. It deletes Realtime Database data and Auth users.
+  Do not run against production projects.
+- Keep changes focused and follow existing code style.
+
+## Thanks
+
+Support the project’s development and keep it sustainable by becoming a [GitHub Sponsor](https://github.com/sponsors/jeromegamez).