Mention sample program execution with Linux framebuffer in README.md

Author: shengwen-tw

README.md

@@ -58,11 +58,14 @@ benefiting the entire application stack.
 `Mado` is built with a minimalist design in mind. However, its verification
 relies on certain third-party packages for full functionality and access to all
 its features. To ensure proper operation, the development environment should
-have the [SDL2 library](https://www.libsdl.org/), [libjpeg](https://www.ijg.org/), and [libpng](https://github.com/pnggroup/libpng) installed.
+have the [libjpeg](https://www.ijg.org/), and [libpng](https://github.com/pnggroup/libpng) installed.
+The [SDL2 library](https://www.libsdl.org/) is optional and only needed if the
+user chooses to use SDL as the backend over the Linux framebuffer.
 * macOS: `brew install sdl2 jpeg libpng`
 * Ubuntu Linux / Debian: `sudo apt install libsdl2-dev libjpeg-dev libpng-dev`
 
-Configure via [Kconfiglib](https://pypi.org/project/kconfiglib/)
+Configure via [Kconfiglib](https://pypi.org/project/kconfiglib/), you should select either SDL
+video output or the Linux framebuffer.
 ```shell
 $ make config
 ```
@@ -72,13 +75,20 @@ Build the library and demo program.
 $ make
 ```
 
-Run sample `Mado` program:
+Run sample `Mado` program with SDL backend:
 ```shell
 $ ./demo-sdl
 ```
 
 Once the window appears, you should be able to move the windows and interact with the widgets.
 
+Run sample `Mado` program with the Linux framebuffer backend:
+```shell
+$ sudo FRAMEBUFFER=/dev/fb0 ./demo-fbdev
+```
+
+If the `FRAMEBUFFER` environment variable is not set, `Mado` will use `/dev/fb0` by default.
+
 ## License
 
 `Mado` is available under a MIT-style license, permitting liberal commercial use.