feat: add README with app description and keybindings

README.md

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+# MM - Minimal Image Viewer
+
+A cross platform minimal image viewer with vim-like keybindings, built using rust and gpui
+
+
+### Keyboard Shortcuts
+
+| Key | Action |
+|-----|--------|
+| `q` or `Escape` | Quit the application |
+| `j` or `→` | Next image |
+| `k` or `←` | Previous image |
+| `g` | Go to first image |
+| `Shift+G` | Go to last image |
+| `c` | Cycle through object fit modes (Contain → Cover → Fill) |