Added documentation for Bluetooth magnetometer service

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

@@ -0,0 +1,38 @@
+# Bluetooth Magnetometer Service 
+
+![](/static/bluetooth/Bluetooth_SIG.png)
+
+The Bluetooth magnetometer service allows another device such as a smartphone to wirelessly receive data from the micro:bit's magnetometer. The magnetometer measures the strength and direction of magnetic fields including the earth's and so it can be used as a digital compass and indicate the way the micro:bit is pointing relative to magnetic north.
+
+Using the Bluetooth magnetometer service you could, for example, create a smartphone application which displays your direction of travel, updating it in real time.   
+
+No additional code is needed on the micro:bit to use the Bluetooth magnetometer service from another device.
+
+~~~~sig
+bluetooth.startMagnetometerService();
+~~~~
+
+### Example: Starting the Bluetooth magnetometer service
+
+The following code shows the Bluetooth magnetometer service being started:
+
+~~~~blocks
+bluetooth.startMagnetometerService();
+~~~~
+
+### Video - Magnetometer service demo
+
+[![micro:bit Bluetooth demo video](/static/bluetooth/microbit_magnetometer.png)](
+    http://www.youtube.com/watch?v=C_0VL4Gp4_U "Click to launch YouTube video"
+    )
+
+### Advanced
+ 
+For more advanced information on the micro:bit Bluetooth magnetometer service including information on using a smartphone, see the [Lancaster University micro:bit runtime technical documentation](http://lancaster-university.github.io/microbit-docs/ble/magnetometer-service/)
+
+### See also
+
+[Bluetooth SIG](https://www.bluetooth.com)
+
+[Bluetooth on micro:bit resources](http://bluetooth-mdw.blogspot.co.uk/p/bbc-microbit.html)
+