Document extension packaging with a tiny sample extension

[culpen90]

Why

Quartz supports installing Chromium-format WebExtensions from folders, .zip archives, .crx packages, and Chrome Web Store listings. The README explains this from a user perspective, but contributors and extension authors would benefit from a tiny working sample and packaging notes.

Where to start

• README.md
• CONTRIBUTING.md
• A new docs file such as docs/extensions.md
• A small sample extension directory, if desired

Suggested approach

Add a short extension-author guide with a minimal Manifest V3 extension that can be loaded in Quartz. Include how to install it as an unpacked folder and how to zip it with manifest.json at the archive root. Mention the macOS 15.4+ requirement for WebExtension support.

Acceptance criteria

• Docs include a minimal working extension example or link to one in the repo.
• Docs explain folder and .zip install expectations clearly.
• Docs mention Chrome Web Store install support and the macOS version requirement.
• The sample extension has a clear, harmless behavior such as a popup or page badge.
• swift build succeeds if project files are touched.

Testing

• Follow the docs from a clean checkout.
• Install the sample extension in Quartz.
• Confirm the sample extension appears in the Extensions menu/status UI.