Add contribution guidelines

CONTRIBUTING.md

@@ -0,0 +1,90 @@
+# Contributing to Loopback Server
+
+Contributions are welcome — bug fixes, features, docs. This is a small project
+with one maintainer, so the notes below are mostly about keeping changes easy
+to review and safe for the people already running a server.
+
+## Before you start
+
+- **Small fixes:** open a pull request directly.
+- **Features, or anything that changes a default, the API or the database
+  schema:** open an issue first so we can agree on the shape before you write
+  the code.
+
+## Running the checks
+
+CI runs three jobs on every pull request; all three must pass before a merge.
+You can run the same checks locally.
+
+**Backend** — the suite needs PostgreSQL and `pg_dump`, both of which the
+compose stack provides. It creates and uses its own throwaway
+`training_api_test` database.
+
+```bash
+make up
+docker compose exec app python -m pytest
+```
+
+**MCP server** — no database or backend needed.
+
+```bash
+cd mcp
+uv sync --locked --no-install-project --extra test --python 3.13
+.venv/bin/python -m pytest -q
+```
+
+**Frontend** — `build` runs the typecheck and the production build.
+
+```bash
+cd frontend
+npm ci
+npm run build
+npm run lint
+```
+
+CI on a first-time contributor's pull request waits for the maintainer to
+approve the run, so don't worry if no checks appear straight away.
+
+## Conventions
+
+- **The API is metric.** Distances are meters, pace is seconds per km, weight
+  is kg. Unit conversion is a display concern and stays in the client.
+- **The iOS app is the other half of the API contract.** A change to a wire
+  format needs a matching change in
+  [loopback-training-app](https://github.com/aderaaij/loopback-training-app);
+  say so in the pull request.
+- **JSON casing differs per resource, on purpose.** Some resources are
+  camelCase, some snake_case; `frontend/src/lib/types.ts` mirrors the API
+  exactly. Follow the casing of the resource you are touching and don't
+  normalize one side without the other.
+- **Existing installs keep working unchanged.** A new setting needs a default
+  that preserves current behaviour. Document it in the root `.env.example`,
+  the README "Configuration" table, and pass it through in
+  `docker-compose.yml`.
+- **Schema changes go through Alembic.** Create the migration with
+  `make create_migration m="..."` and commit it with the model change.
+- **Dependencies are installed from the lockfiles.** If you change
+  `pyproject.toml` or `package.json`, commit the updated `uv.lock` or
+  `package-lock.json` too — CI fails when they drift.
+
+## Changelog
+
+User-visible changes get an entry in `CHANGELOG.md`. Add yours to the existing
+`## [Unreleased]` section at the top, under the fitting heading (`Added`,
+`Changed`, `Fixed`); create the section only if it isn't there. Don't bump the
+version — that happens when a release is cut.
+
+## Pull requests
+
+- One change per pull request; unrelated fixes are easier to review apart.
+- Say what the change does, why, and how you tested it.
+- Add or update tests for backend and MCP changes.
+- Keep the branch up to date with `main` if it conflicts.
+
+`main` is what the `latest` Docker image is built from, so code only lands
+there through a pull request with green CI.
+
+## License
+
+By contributing you agree that your contribution is licensed under the
+project's [MIT License](LICENSE).

README.md

@@ -439,6 +439,8 @@ setup doesn't advertise.
 
 ## Development
 
+Want to contribute? See [CONTRIBUTING.md](CONTRIBUTING.md).
+
 ```bash
 make up       # Start containers
 make down     # Stop containers