docs: add contribution guide

Author: engineervix

CONTRIBUTING.md

@@ -0,0 +1,57 @@
+# Contributing to Kwelea
+
+## Prerequisites
+
+- Go (see `go.mod` for the minimum version)
+- [just](https://just.systems) -- task runner
+- [lefthook](https://github.com/evilmartians/lefthook) -- git hooks
+- [gotestsum](https://github.com/gotestyourself/gotestsum) -- test runner
+
+```bash
+go install gotest.tools/gotestsum@latest
+lefthook install
+```
+
+## Development workflow
+
+```bash
+just build        # compile the binary to bin/
+just test         # run tests
+just check        # format + vet
+just serve        # build and serve kwelea's own docs (dogfood)
+```
+
+Pre-commit hooks run `fmt`, `vet`, and `test` on changed `.go` files.
+
+## Making changes
+
+For non-trivial changes, open an [issue](https://github.com/engineervix/kwelea/issues) first to discuss the approach. Bug fixes and typos can go straight to a PR.
+
+If you are proposing a feature, explain what problem it solves and keep the scope narrow — a focused PR is easier to review and more likely to land.
+
+1. Fork the repo and create a branch (`feat/my-thing`, `fix/my-bug`).
+2. Write tests for new behaviour. Existing tests live in `internal/*/` alongside the code.
+3. Update `docs/` alongside the code if the change affects users.
+4. Open a pull request against `main`.
+
+## Commit format
+
+This project uses [Conventional Commits](https://www.conventionalcommits.org). The release changelog is generated automatically from commit messages, so the format matters.
+
+```
+feat: add search highlighting for multi-word queries
+fix: resolve port conflict on dev server restart
+docs: clarify nav ordering in configuration guide
+chore: bump D2 to v0.7.2
+```
+
+Types: `feat`, `fix`, `docs`, `test`, `refactor`, `chore`.
+Breaking changes: append `!` after the type (`feat!:`) and describe the break in the commit body.
+
+## Running the full test suite
+
+```bash
+just test-v          # verbose output
+```
+
+CI runs on Linux and macOS. Windows builds are cross-compiled in CI -- flag any platform-specific issues in your PR.