Add demo.

README.md

@@ -8,3 +8,39 @@ This makes it difficult to do things like displaying information in the backgrou
 `i3bgwin` provides a way to run any application in the background behind all other windows,
 provided that they support embedding into another window, e.g. `urxvt -embed <window>` or
 `xterm -into <window>`.
+
+## Usage
+
+```sh
+$ i3bgwin command...
+```
+
+`i3bgwin` will replace `{windowid}` with the X11 window ID that the child process should embed
+into.
+
+For example, to run `urxvt` inside, do `i3bgwin urxvt -embed {windowid}`, and to run `xterm`,
+do `i3bgwin xterm -into {windowid}`.
+
+
+## Demo
+
+![Demo of i3bgwin with the visualizer Cava](./screenshots/cava-demo.png)
+
+Here, we run [Cava][https://github.com/karlstav/cava] inside `rxvt` on the i3 background window.
+
+The command used is:
+
+```sh
+$ i3bgwin rxvt -depth 32 -bg '[00]black' --color6 '[50]cyan' +sb -embed {windowid} -e cava
+```
+
+## Building
+
+With `gcc` and X11 development files (`libx11-dev` and `libxext-dev` on Debian/Ubuntu) installed,
+simply build with
+
+```sh
+$ make
+```
+
+`i3bgwin` will be created in the current directory. You can copy it anywhere you wish.