Update nRF52840 README.md (#1796)

* Update nRF52840 README.md

* Separate diagnostic module descriptin to DIAG.md file

examples/platforms/nrf52840/DIAG.md

@@ -0,0 +1,89 @@
+## Diagnostic module
+
+nRF52840 port extends [OpenThread Diagnostics Module][DIAG].
+
+New commands allow for more accurate low level radio testing.
+
+### New commands
+ * [diag id](#diag-id)
+ * [diag listen](#diag-listen)
+ * [diag transmit](#diag-transmit)
+
+### Diagnostic radio packet
+[diag listen](#diag-listen) and [diag transmit](#diag-transmit) use radio frame payload specified below.
+
+ ```c
+ struct PlatformDiagMessage
+ {
+     const char mMessageDescriptor[11];
+     uint8_t mChannel;
+     int16_t mID;
+     uint32_t mCnt;
+ };
+ ```
+
+`mMessageDescriptor` is constant string `"DiagMessage"`.<br />
+`mChannel` contains channel number on which packet was transmitted.<br />
+`mID` contains board ID set with [diag id](#diag-id) command.<br />
+`mCnt` is a counter incremented every time board transmits diagnostic radio packet.
+
+If [listen](#diag-listen) mode is enabled and OpenThread was built with `DEFAULT_LOGGING` flag, JSON string is printed every time diagnostic radio packet is received.
+
+```JSON
+ {"Frame":{
+   "LocalChannel":"<listening board channel>",
+   "RemoteChannel":"<mChannel>",
+   "CNT":"<mCnt>",
+   "LocalID":"<listening board ID>",
+   "RemoteID":"<mID>",
+   "RSSI":"<packet RSSI>"
+ }}
+```
+
+### diag id
+Get board ID.
+
+### diag id \<id\>
+Set board ID.
+
+Value range 0 to 32767.
+
+Default: `-1`.
+
+### diag listen
+Get listen state.
+
+### diag listen \<listen\>
+Set listen state.
+
+`0` disable listen state.<br />
+`1` enable listen state.
+
+Default: listen disabled.
+
+### diag transmit
+Get messages count and interval between them that will be transmitted after `diag transmit start`.
+
+### diag transmit interval \<interval\>
+Set interval in ms between transmitted messages.
+
+Value range 1 to 4294967295.
+
+Default: `1`.
+
+### diag transmit count \<count\>
+Set number of messages to be transmitted.
+
+Value range 1 to 2147483647<br />
+or<br />
+`-1` continuous transmission.
+
+Default: `1`
+
+### diag transmit stop
+Stop ongoing transmission regardless of remaining number of messages to be sent.
+
+### diag transmit start
+Start transmiting messages with specified interval.
+
+[DIAG]: ./../../../src/diag/README.md

examples/platforms/nrf52840/README.md

@@ -37,8 +37,7 @@ part of the [nRF5x Command Line Tools][nRF5x-Command-Line-Tools].
 [nRF5x-Command-Line-Tools]: https://www.nordicsemi.com/eng/Products/nRF52840#Downloads
 
 ```bash
-$ nrfjprog -f nrf52 --chiperase --program output/nrf52840/bin/ot-cli-ftd.hex
+$ nrfjprog -f nrf52 --chiperase --program output/nrf52840/bin/ot-cli-ftd.hex --reset
-$ nrfjprog -f nrf52 -r
 ```
 
 ## Running the example
@@ -108,7 +107,7 @@ $ nrfjprog -f nrf52 -r
 
 For a list of all available commands, visit [OpenThread CLI Reference README.md][CLI].
 
-[CLI]: https://github.com/openthread/openthread/blob/master/src/cli/README.md
+[CLI]: ./../../../src/cli/README.md
 
 ## Logging module
 
@@ -132,6 +131,15 @@ Due to a known issue in Segger’s J-Link firmware, depending on your version, y
 
 Run the following command: `MSDDisable`
 
+## Diagnostic module
+
+nRF52840 port extends [OpenThread Diagnostics Module][DIAG].
+
+You can read about all the features [here][nRFDIAG].
+
+[DIAG]: ./../../../src/diag/README.md
+[nRFDIAG]: DIAG.md
+
 ## Radio driver documentation
 
 The radio driver comes with documentation that describes the operation of state