docs: add CONTRIBUTING.md (#42)

CONTRIBUTING.md

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+# Contributing to Forji
+
+Thanks for your interest in contributing! Forji is a native iOS client for
+Forgejo, and contributions are welcome. These guidelines keep changes easy to
+review and the codebase easy to maintain for the next person.
+
+## Pull requests
+
+- **One concern per pull request.** Keep each PR focused on a single change.
+  Split bug fixes out from refactors. A behavior-preserving refactor is easy to
+  approve on its own; bundling it with a fix makes the actual fix hard to
+  isolate and review.
+- **Keep the description accurate.** The PR description must match what the diff
+  actually does. If you change the approach while iterating, update the
+  description to match the final code, and explain *why* the change fixes the
+  problem.
+- **Prefer small, stacked PRs** over one large change. For anything large, open
+  an issue first so we can agree on the shape before you write the code.
+- Respond to review feedback by fixing it or explaining it.
+
+## Reporting bugs
+
+Open an issue with exact reproduction steps so the bug can actually be
+reproduced and fixed:
+
+- Device and iOS version.
+- What to tap and what to watch for.
+- Expected behavior vs. what actually happens.
+
+## Architecture
+
+New code follows MVVM with Apple-native tools:
+
+- `@Observable` view models with `async/await`.
+- **No external state-management dependency.** The project deliberately stays on
+  Apple's `@Observable` + `async/await` direction rather than adopting a
+  third-party architecture.
+- Keep the code easy to understand for the next contributor. Favor the simplest
+  approach that fits Apple's idioms.
+- ForgejoKit holds the platform-agnostic API logic; the Forji app holds the
+  SwiftUI views, authentication, and persistence. Keep that separation.
+
+## Code style
+
+The repository ships its own formatter and linter configs
+([`.swiftformat`](.swiftformat), [`.swiftlint.yml`](.swiftlint.yml)). Match them
+before opening a PR, and match the existing style in the files you touch:
+
+```bash
+just format   # SwiftFormat
+just lint     # SwiftLint
+```
+
+## Tests
+
+- Include unit tests for the behavior you add or change.
+- When you fix a UI behavior that has no automated coverage, add a UI regression
+  test. If a change is hard to cover, say so in the PR and offer a UI test.
+- See the [README](README.md) for `just test`, `just test-ui`, and the
+  integration test setup.
+
+## License and the App Store exception
+
+Forji is licensed under the [GNU General Public License v3.0](LICENSE) with an
+[App Store Exception](LICENSE#app-store-exception) that permits distribution of
+the compiled app through Apple's App Store.
+
+By submitting a pull request you agree your contribution is licensed under
+GPLv3 and may be distributed through the App Store under that exception. To
+confirm, include this sentence in your PR description or a comment:
+
+> "I grant Stefan Hausotte an irrevocable, worldwide, royalty-free license to
+> use, sublicense, and distribute my contribution, including through Apple's App
+> Store under the project's App Store exception."
+
+You keep your copyright, the project stays open source under GPLv3, and you will
+be credited as a contributor.