Add BLE File Transfer as a submodule and update drivers.rst

Author: prcutler

.gitmodules

@@ -920,3 +920,6 @@
 [submodule "libraries/helpers/ble_beacon"]
 	path = libraries/helpers/ble_beacon
 	url = https://github.com/adafruit/Adafruit_CircuitPython_BLE_Beacon.git
+[submodule "libraries/helpers/ble_file_transfer"]
+	path = libraries/helpers/ble_file_transfer
+	url = https://github.com/adafruit/Adafruit_CircuitPython_BLE_File_Transfer.git

docs/drivers.rst

@@ -147,6 +147,7 @@ Helpers for Bluetooth Low Energy (BLE).
     BLE Magic Light <https://circuitpython.readthedocs.io/projects/ble_magic_light/en/latest/>
     BLE MIDI <https://circuitpython.readthedocs.io/projects/ble_midi/en/latest/>
     BLE Radio <https://circuitpython.readthedocs.io/projects/ble_radio/en/latest/>
+    BLE File Transfer <https://docs.circuitpython.org/projects/ble_file_transfer/en/latest/>
 
 LoRa Wireless Helpers
 ^^^^^^^^^^^^^^^^^^^^^