Add demo video section to README

- Added Demo section showcasing ColorMNet colorization results
- Added example video link (Marx Brothers scene, 850+ frames)
- Updated .gitignore to exclude large video files from examples/
- Videos will be hosted via GitHub releases or external hosting

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.gitignore

@@ -23,6 +23,11 @@ checkpoints/
 data/*.pth
 *.zip
 
+# Examples (too large for git - host externally)
+examples/*.mp4
+examples/*.avi
+examples/*.mov
+
 # Test and helper scripts
 test_*.py
 install_*.py