Added documentation for Bluetooth button service

docs/reference/bluetooth/start-button-service.md

@@ -0,0 +1,42 @@
+# Bluetooth Button Service 
+
+![](/static/bluetooth/Bluetooth_SIG.png)
+
+The Bluetooth button service makes it possible for another device such as a smartphone to be notified wirelessly whenever a button on the front of a micro:bit is pressed. Each of the two micro:bit buttons can be in one of three possible states:
+
+* Not pressed
+* Pressed
+* Long press - pressed and held down for at least 2 seconds  
+
+The button service allows you to make other things which are connected to your micro:bit using Bluetooth respond in some way when you press either of the buttons. You could, for example, hide your smartphone somewhere in the room and have it make an amusing noise when you press either of the buttons on your micro:bit. We'll leave it to you to decide what would be amusing.
+
+No additional code is needed on the micro:bit to use the Bluetooth button service from another device.
+
+~~~~sig
+bluetooth.startButtonService();
+~~~~
+
+### Example: Starting the Bluetooth button service
+
+The following code shows the Bluetooth button service being started:
+
+~~~~blocks
+bluetooth.startButtonService();
+~~~~
+
+### Video - Button service demo - Starts at 0:59
+
+[![micro:bit Bluetooth demo video](/static/bluetooth/microbit_button.png)](
+    http://www.youtube.com/watch?v=aep_GVowKfs "Click to launch YouTube video"
+    )
+
+### Advanced
+ 
+For more advanced information on the micro:bit Bluetooth button service including information on using a smartphone, see the [Lancaster University micro:bit runtime technical documentation](http://lancaster-university.github.io/microbit-docs/ble/button-service/)
+
+### See also
+
+[Bluetooth SIG](https://www.bluetooth.com)
+
+[Bluetooth on micro:bit resources](http://bluetooth-mdw.blogspot.co.uk/p/bbc-microbit.html)
+