Add higan command-line docs.

docs/interface/higan-cli.md

@@ -1,5 +1,35 @@
-TODO
 
-higan accepts a path to a game folder,
-and also the `--fullscreen` option.
+# Synopsis
 
+> higan [*\-\-fullscreen*] [*PATH*]
+
+# Description
+
+When launched with `--fullscreen`,
+higan will automatically enter full-screen mode
+when it starts.
+This is not much use unless you also specify `PATH`,
+because you won't be able to load a game
+until you exit full-screen mode
+by pressing the "Toggle Fullscreen"
+[hotkey](higan-config.md#hotkeys).
+
+When `PATH` is the path to an existing
+[game folder](../concepts/game-folders.md)
+for any supported console,
+that game will be loaded when higan starts.
+
+When `PATH` is the path to a ROM file
+for any supported console,
+it will be imported into a game folder in
+[the Game Library](../concepts/game-library.md),
+and then loaded from there when higan starts.
+
+# Examples
+
+Play a previously-imported copy of Super Mario World
+in full-screen (assuming Linux defaults):
+
+```sh
+higan --fullscreen ~/Emulation/"Super Famicom"/"Super Mario World.sfc"
+```