Add write-value-to-serial documentation

docs/reference/radio/write-value-to-serial.md

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+# Write Value To Serial
+
+Writes the full data received data  via ``radio`` to serial in JSON format.   
+**Note** - This method only works for [send number](/reference/radio/send-number) and [send value](/reference/radio/send-value). It does not work for [send string](/reference/radio/send-string) (although a string can be sent with [send value](/reference/radio/send-value)).   
+
+## Data received format
+The format for received data printed to serial is as follows    
+- [send number](/reference/radio/send-number) - ```{v:ValueSent,t:MicrobitTimeAlive,s:Unused}```
+- [send value](/reference/radio/send-number) - ```{v:Value,t:MicrobitTimeAlive,s:Unused,n:"Name"}```
+- [send string](/reference/radio/send-string) - ```{}``` (currently unavailable)
+
+## Important Security Consideration
+
+The functions in the ``radio`` namespace allow the BBC micro:bit to communicate with other micro:bits.
+
+This API does not contain any form of encryption, authentication or authorization. It's purpose is solely for use as a teaching aid to demonstrate how simple communications operates, and to provide a sandpit through which learning can take place.
+
+For serious applications, BLE should be considered a substantially more secure alternative.
+
+```sig
+radio.writeValueToSerial()
+```
+
+### Parameters
+
+* None
+
+### Examples
+
+When ```radio``` data is received (after pressing A button on 2nd micro:bit), output temperature data to serial.
+
+```blocks
+input.onButtonPressed(Button.A, () => {
+    radio.sendNumber(input.temperature());
+});
+radio.onDataReceived(() => {
+    radio.writeValueToSerial();
+});
+```
+Example output to serial when A button pressed:   
+```{v:27,t:323,s:0}```
+
+### See also
+
+[send number](/reference/radio/send-number), [send value](/reference/radio/send-number), [on data received](/reference/radio/on-data-received)