docs: add contributing guidelines

CONTRIBUTING.md

@@ -0,0 +1,57 @@
+# Contributing to TREK
+
+Thanks for your interest in contributing! Please read these guidelines before opening a pull request.
+
+## Ground Rules
+
+1. **Ask in Discord first** — Before writing any code, pitch your idea in the `#dev-discussion` channel on our [Discord server](https://discord.gg/P7TUxHJs). We'll let you know if the PR is wanted and give direction. PRs that show up without prior discussion will be closed
+2. **One change per PR** — Keep it focused. Don't bundle unrelated fixes or refactors
+3. **No breaking changes** — Backwards compatibility is non-negotiable
+4. **Target the `dev` branch** — All PRs must be opened against `dev`, not `main`
+5. **Match the existing style** — No reformatting, no linter config changes, no "while I'm here" cleanups
+
+## Pull Requests
+
+### Your PR should include:
+
+- **Summary** — What does this change and why? (1-3 bullet points)
+- **Test plan** — How did you verify it works?
+- **Linked issue** — Reference the issue (e.g. `Fixes #123`)
+
+### Your PR will be closed if it:
+
+- Wasn't discussed and approved in `#dev-discussion` on Discord first
+- Introduces breaking changes
+- Adds unnecessary complexity or features beyond scope
+- Reformats or refactors unrelated code
+- Adds dependencies without clear justification
+
+### Commit messages
+
+Use [conventional commits](https://www.conventionalcommits.org/):
+
+```
+fix(maps): correct zoom level on Safari
+feat(budget): add CSV export for expenses
+```
+
+## Development Setup
+
+```bash
+git clone https://github.com/mauriceboe/TREK.git
+cd TREK
+
+# Server
+cd server && npm install && npm run dev
+
+# Client (separate terminal)
+cd client && npm install && npm run dev
+```
+
+Server: `http://localhost:3001` | Client: `http://localhost:5173`
+
+On first run, check the server logs for the auto-generated admin credentials.
+
+## More Details
+
+See the [Contributing wiki page](https://github.com/mauriceboe/TREK/wiki/Contributing) for the full tech stack, architecture overview, and detailed guidelines.