doc: add link to firmware source code to README.md

Signed-off-by: Rafał Kuźnia <rafal.kuznia@nordicsemi.no>

README.md

@@ -15,3 +15,7 @@ The software provided has been tested with the nRF52840 DK and the nRF52840 Dong
 
 To start using the nRF Sniffer, you must program the firmware, install Wireshark, and configure the nRF Sniffer capture plugin.
 See Nordic Semiconductor's [nRF Sniffer for 802.15.4 user guide](https://infocenter.nordicsemi.com/topic/ug_sniffer_802154/UG/sniffer_802154/intro_802154.html) for detailed instructions and complete documentation.
+
+## Firmware source code
+
+The source code of the nRF Sniffer for 802.15.4 firmware is available in the [nRF Connect SDK](https://github.com/nrfconnect/sdk-nrf/tree/v2.6.0/samples/peripheral/802154_sniffer).