Add instructions for upgrading from V2 to V3 in README (#197)

This commit adds a new section in the README.md file detailing instructions for users upgrading from V2 to V3.

README.md

@@ -60,6 +60,11 @@ Follow the programming instruction on page 8 of the [Quick Start Guide](https://
 
 > Note: There are also physical reset buttons on both keyboards which can be used to enter and exit the bootloader mode. Their location is described in section 2.7 on page 9 in the [User Manual](https://kinesis-ergo.com/wp-content/uploads/Advantage360-ZMK-KB360-PRO-Users-Manual-v3-10-23.pdf) and use is described in section 5.9 on page 14. 
 
+### Upgrading from V2 to V3
+
+If you are upgrading from V2 to V3, and if the flashing didn't work as expected (i.e. if you are unable to pair the keyboard via Bluetooth), then consider [resetting](https://kinesis-ergo.com/support/kb360pro/#firmware-updates) both halves of the keyboard to its native state. Make sure to use the `settings-reset.uf2` file from 
+the V3 branch of this repository. After doing this, proceed with the flashing instructions above.
+
 ## Other support
 
 Further support resources can be found on Kinesis.com: