docs: add CONTRIBUTING.md

Covers contribution workflow, PR expectations, code quality rules,
three-environment deployment model, and the requirement to open and
discuss an issue before submitting a PR.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: lgelauff

CONTRIBUTING.md

@@ -0,0 +1,60 @@
+# Contributing to Montage
+
+Montage is a small project maintained by volunteers. Contributions are welcome — please read this before opening a PR.
+
+We follow the [Wikimedia technical spaces Code of Conduct](https://www.mediawiki.org/wiki/Code_of_Conduct).
+
+## Getting started
+
+Check the [open issues](https://github.com/hatnote/montage/issues) for something to pick up. Issues tagged [`good-first-issue`](https://github.com/hatnote/montage/issues?q=is%3Aissue+is%3Aopen+label%3Agood-first-issue) are a good starting point.
+
+Before writing any code, leave a comment on the issue to let maintainers know you're working on it and to confirm the approach. If no issue exists for what you want to do, open one first. **Do not open a PR without a corresponding issue that has received a response from a maintainer** — especially if you are a new contributor. This avoids wasted effort on both sides.
+
+See [dev.md](dev.md) for local setup instructions.
+
+## Before opening a PR
+
+Follow the setup instructions in [dev.md](dev.md), including installing pre-commit hooks and running tests locally. Make sure tests pass before opening a PR.
+
+## Pull request expectations
+
+- CI must pass
+- One focused change per PR — link the issue it addresses (`Fixes #NNN`)
+- If your PR adds or changes database columns, include the SQL migration and flag it in the PR description
+- Do not commit screenshots, generated files, or large binaries
+
+## Code quality
+
+- Follow existing patterns in the file you're editing
+- Backend: read `montage/rdb.py` before writing queries — column names are authoritative there. Note that `.filter()` and `.order_by()` return new objects — always reassign
+- Backend: write code that works with both MySQL (production) and SQLite (tests) — no MySQL-only syntax
+- Frontend: use Composition API with `<script setup>` — no Options API
+- All user-facing strings must go in `frontend/src/i18n/en.json` and use `t('key')` — TranslateWiki handles other locales, do not edit them manually
+
+## When to open a PR for review
+
+Only open a PR for review when you consider it ready to merge. If your work is still in progress, open it as a **draft PR** — maintainers will not review draft PRs.
+
+A PR is ready for review when:
+- It does one thing and does it completely
+- CI passes
+- You have tested it yourself and are confident it works
+- You would be comfortable if it were merged as-is
+
+Do not open a PR to ask for direction or feedback on an approach — use the issue for that conversation first.
+
+## Deployment
+
+Only maintainers have access to the three Toolforge environments:
+
+| Environment | Purpose |
+|-------------|---------|
+| montage-dev | Flexible test environment — maintainers can deploy any branch here |
+| montage-beta | Tracks `master` — used by volunteers to catch bugs before production |
+| montage-prod | Stable production — promoted manually when beta is stable |
+
+Contributors do not need Toolforge access. Push your branch to GitHub and open a PR against `master`. A maintainer can then deploy your branch to montage-dev to test it — either in isolation or combined with other PRs to check interaction effects.
+
+## Questions
+
+Leave a comment on the relevant GitHub issue, or reach out on [Commons talk:Montage](https://commons.wikimedia.org/wiki/Commons_talk:Montage).