Add Built with Markdown Viewer section: Markdown Desk (macOS native wrapper)

[jhrepo]

Hi @ThisIs-Developer 👋

I built Markdown Desk, a native macOS desktop app that wraps Markdown Viewer using Tauri.

It adds a few desktop-specific features on top of your excellent web app:

• Live Reload — watches the opened file and refreshes on external edits
• Native file open/save — Cmd+O / Cmd+S
• Tracks upstream — includes your source as a Git submodule and periodically merges updates into new app releases

The original Markdown Viewer source is included as a Git submodule and will not be modified — all customizations are applied through a separate bridge script.

This PR simply adds a small "Built with Markdown Viewer" section to the README so users who prefer a native experience can discover it.

Thank you for creating Markdown Viewer — it's a great project! 🙏

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