Mention Linux framebuffer mode in README.md

Author: shengwen-tw

README.md

@@ -71,13 +71,28 @@ Build the library and demo program.
 $ make
 ```
 
-Run sample `Mado` program:
+`Mado` supports SDL or Linux framebuffer as backends. The basic usage is given as:
+
 ```shell
-$ ./demo-sdl
+$ ./demo -b backend [-p fb-path]
+```
+
+For example, to run `Mado` with SDL as backend:
+
+```
+$ ./demo -b sdl
 ```
 
 Once the window appears, you should be able to move the windows and interact with the widgets.
 
+To run `Mado` with Linux framebuffer as backend, type:
+
+```
+$ ./demo -b fb -p /dev/fb0
+```
+
+Note that the user may require to execute `Mado` with `sudo` command with Linux framebuffer mode.
+
 ## License
 
 `Mado` is available under a MIT-style license, permitting liberal commercial use.