Add note to Bluetooth readme about link keys (#508)

Fixes raspberrypi/pico-sdk#1457

pico_w/bt/README.md

@@ -16,3 +16,7 @@ Define `WANT_HCI_DUMP=1` in your CMakeLists.txt file. Uncomment this line to ena
 
 To view packet logs, save the output from the debug port (e.g. the uart) to a file and afterwards run `pico-sdk/lib/btstack/tool/create_packet_log.py <filename>`.
 This will generate a file with the same name except for a `pklg` extension. This can be opened in the [Wireshark](https://www.wireshark.org) application to analyze communications activity.
+
+## Link keys
+
+By default, the last two sectors of flash are used to store Bluetooth link keys and this relies on the Bluetooth address. Old pre-release versions of the SDK had issues with some devices using a default Bluetooth address. If you experience issues with pairing try Resetting flash memory, see https://www.raspberrypi.com/documentation/microcontrollers/raspberry-pi-pico.html#resetting-flash-memory