Add a focused contributor guide

CHANGELOG.md

@@ -6,6 +6,11 @@ All notable changes to kage are recorded here. The format follows
 
 ## [Unreleased]
 
+### Added
+
+- `CONTRIBUTING.md` documents the development setup, test commands, and
+  expectations for focused pull requests.
+
 ### Fixed
 
 - `--resume` picks an interrupted crawl back up instead of doing nothing ([#36](https://github.com/tamnd/kage/issues/36)).

CONTRIBUTING.md

@@ -0,0 +1,53 @@
+# Contributing to kage
+
+Thanks for helping. kage is a small, opinionated tool: patches that stay focused
+and come with tests land fastest.
+
+## Development setup
+
+You need a recent Go toolchain (the `go` version in `go.mod`; Go 1.21+ will
+download it automatically) and, for browser tests, Chrome or Chromium.
+
+```bash
+git clone https://github.com/tamnd/kage
+cd kage
+make build          # -> bin/kage
+make test-short     # unit tests, no Chrome
+make test           # full suite, including Chrome e2e when a browser is present
+make vet
+```
+
+Point kage at a browser with `KAGE_CHROME` / `CHROME_BIN` or `--chrome` if it is
+not on the default path.
+
+## What makes a good PR
+
+- **One concern per PR.** Fix install, or sanitize, or packing — not all three
+  unless they are inseparable.
+- **Tests for the regression.** Prefer a small unit test over a full site clone.
+  Chrome-driven tests should skip under `-short` and when no browser is found.
+- **Update `CHANGELOG.md`** under `[Unreleased]` when the change is user-visible.
+- **Match the local style.** Packages are deep modules with package comments;
+  exported behaviour is described in prose, not only in names. Run `gofmt -s`.
+- **No new dependencies** unless they remove more code than they add. Explain
+  why a new module belongs in kage and what local code it replaces.
+
+## Suggested first contributions
+
+Open issues are the best map. High-impact areas that have already bitten users:
+
+- SPA / lazy-content wait strategies for sites that never finish loading.
+- Windows CI for `-short` tests.
+
+Please open an issue before large design changes so the approach can be agreed.
+
+## Reporting bugs
+
+Include the kage version (`kage version`), OS, the exact command, and a minimal
+reproducing URL when possible. Logs from a failed page render are especially
+useful.
+
+## License
+
+By contributing, you agree that your contributions are licensed under the MIT
+License (see [LICENSE](LICENSE)).