Add note about UF2 bootloader being required

https://forums.adafruit.com/viewtopic.php?f=60&t=158279&p=803678#p803678 requested a prominent note about this.

Author: dhalbert

_board/arduino_nano_33_ble.md

@@ -11,6 +11,8 @@ board_image: "arduino_nano_33_ble.jpg"
 This compact and reliable NANO board is built around the NINA B306 module, based on Nordic nRF 52840 and containing a powerful Cortex M4F. Its architecture, fully compatible with Arduino IDE Online and Offline, has a 9 axis Inertial Measurement Unit (IMU) and a reduced power consumption compared to other same size boards.
 This allows the design of wearable devices and movement sensing projects that need to communicate to other devices at a close range. Arduino NANO 33 BLE is also ideal for automation projects thanks to the multiprotocol BT 5.0 radio.
 
+To use this board with CircuitPython you must [replace the existing bootloader](https://forums.adafruit.com/viewtopic.php?f=60&t=158279) with the Adafruit UF2 bootloader. You need to restore the original bootloader to use the board again with Arduino.
+
 ## Purchase
 * [Arduino](https://store.arduino.cc/usa/nano-33-ble)