Add comprehensive README documentation

[Copilot]

The repository lacked clear documentation explaining its purpose and functionality.

Changes

• Project Overview: Added description of YoutubeLookup as a privacy-focused YouTube video browser with duration-based categorization
• Feature Documentation: Documented 8 core features including search, sorting, metadata display, and thumbnail previews
• Usage Guide: Added step-by-step instructions and channel examples
• Technical Details: Documented stack (HTML5/CSS3/JS, YouTube Data API v3) and repository structure
• Data Collection: Explained Python script usage with command examples
• Contributing: Added instructions for adding new channels

The README now clearly communicates that this is a web tool for browsing 25+ tech YouTube channels organized by video duration (0-15min, 15-30min, 30-60min, 1-5hr, >5hr), using pre-downloaded JSON metadata to avoid direct YouTube tracking.

Original prompt

What is this repository about?

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