docs: add gitui extra documentation

extras/gitui/README.md

@@ -0,0 +1,14 @@
+# Usage
+
+Save one of the available theme files as `theme.ron` in the appropriate directory for your operating system:
+
+-   `$HOME/.config/gitui/theme.ron` (MacOS)
+-   `$XDG_CONFIG_HOME/gitui/theme.ron` (Linux using XDG)
+-   `$HOME/.config/gitui/theme.ron` (Linux)
+-   `%APPDATA%\gitui\theme.ron` (Windows)
+
+Alternatively, you can save both the light and dark theme files in the same directory and switch between them by passing the theme name with the `-t` flag:
+
+```sh
+gitui -t cyberdream-light.ron
+```