docs: add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to PayStream
+
+Thank you for contributing!
+
+## Go Module Workflow
+
+1. Fork the repository and clone your fork:
+   ```bash
+   git clone https://github.com/YOUR_USERNAME/paystream.git
+   cd paystream
+   git remote add upstream https://github.com/Breedar/paystream.git
+   ```
+2. Create a feature branch:
+   ```bash
+   git checkout -b feat/your-feature
+   ```
+3. Push your branch and open a pull request against `main`.
+
+## Running Tests
+
+```bash
+# Build all Go packages
+go build ./...
+
+# Run all Go tests
+go test ./...
+```
+
+For the dashboard:
+```bash
+cd web/dashboard
+pnpm install
+pnpm typecheck
+```
+
+## Branch and Commit Conventions
+
+Branch names: `<type>/<short-description>` — e.g. `feat/webhook-retry`, `fix/ci-pnpm`.
+
+Commits follow [Conventional Commits](https://www.conventionalcommits.org/):
+
+```
+<type>(<scope>): <short summary>
+```
+
+Common types: `feat`, `fix`, `docs`, `chore`, `ci`, `refactor`, `test`.
+
+## Submitting a Pull Request
+
+- Keep each PR focused on a single concern.
+- Ensure `go build ./...` and `go test ./...` pass before opening a PR.
+- Reference the related issue with `closes #<number>` in the PR description.