Adding section to README explaining how to update the UNO WIFI REV2 N…

…INA module via SerialNINAPassthrough

README.md

@@ -12,6 +12,13 @@ This firmware uses [Espressif's IDF](https://github.com/espressif/esp-idf)
 1. Load the `Tools -> SerialNINAPassthrough` example sketch on to the board
 1. Use `esptool` to flash the compiled firmware
 
+## Notes
+If updating **Arduino UNO WiFi Rev. 2** NINA firmware via [SerialNINAPassthrough](https://github.com/arduino-libraries/WiFiNINA/blob/master/examples/Tools/SerialNINAPassthrough/SerialNINAPassthrough.ino) sketch then the `esptool` invocation needs to be changed slightly:
+```diff
+-  --baud 115200 --before default_reset
++  --baud 115200 --before no_reset
+```
+
 ## License
 
 Copyright (c) 2018-2019 Arduino SA. All rights reserved.