docs(readme): add a Contributing section and link the roadmap (LeyckerS#78)

The README had 18 sections and no route into contributing: the documentation
table listed nine files but not CONTRIBUTING.md, and the roadmap was not
linked anywhere. With 1,579 stars and one watcher, the README is the only
page most people ever see, so it was the largest gap in the funnel.

Adds a Contributing section pointing at the roadmap (LeyckerS#39), both labels, and
the fact that most open work needs no Windows machine.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

Author: LeyckerS

README.md

@@ -230,10 +230,32 @@ Full write-up: **[docs/ARCHITECTURE.md](docs/ARCHITECTURE.md)**.
 | [Engineering notes](docs/ENGINEERING_NOTES.md) | the measurements behind the design decisions |
 | [Troubleshooting](docs/TROUBLESHOOTING.md) | 403s, Turnstile failures, CDP conflicts, stalls |
 | [FAQ](docs/FAQ.md) | why curl_cffi is mandatory, what `--browsers` means |
+| [Contributing](CONTRIBUTING.md) | architecture, what counts as a contribution, the verification commands |
 | [Changelog](CHANGELOG.md) | every version since 14.0 |
 
 ---
 
+## 🤝 Contributing
+
+**[→ The roadmap](https://github.com/LeyckerS/moondownloader/issues/39)** — everything open, ranked by
+how hard it is and whether it needs Windows.
+
+Most of the open work does **not** require a Windows machine. Documentation, CI, tests and dependency
+work all run on Linux and macOS, and the test suite stubs Chrome and the network at the `moon_extract`
+boundary, so it runs anywhere. Each issue says up front which it is.
+
+| | |
+|:--|:--|
+| [good first issue](https://github.com/LeyckerS/moondownloader/labels/good%20first%20issue) | scoped small, with the files to touch and the acceptance criteria already written out |
+| [help wanted](https://github.com/LeyckerS/moondownloader/labels/help%20wanted) | everything open to outside contributors, including the larger items |
+
+Claim one by commenting on it in your own words — no need to ask permission first. What gets a pull
+request merged, and what gets one sent back, is written out in [CONTRIBUTING.md](CONTRIBUTING.md).
+
+Everyone who has shipped a change is named in [AUTHORS.md](AUTHORS.md) with what they did.
+
+---
+
 ## 🔒 The local server
 
 Binds **127.0.0.1 only**, on a kernel-chosen port, and every `/api/` call must carry the token minted