task-02: small amendment to the README

Author: OlegHahm

task-02/README.md

@@ -27,6 +27,8 @@ static const shell_command_t shell_commands[] = {
 /* ... */
 ```
 
+Please note, that the list of shell commands must be terminated with an empty entry.
+
 ## Task 2.2: Control the hardware
 1.  Include the [`led.h`](http://doc.riot-os.org/led_8h.html) file to get access
     to the `LED0_TOGGLE` macro.