Added feather52840 readme

ports/nrf/boards/feather52840/README.md

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+# Setup
+
+## Installing CircuitPython submodules
+
+Before you can build, you will need to run the following commands once, which
+will install the submodules that are part of the CircuitPython ecosystem, and
+build the `mpy-cross` tool:
+
+```
+$ cd circuitpython
+$ git submodule update --init
+$ make -C mpy-cross
+```
+
+You then need to download the SD and Nordic SDK files via:
+
+> This script relies on `wget`, which must be available from the command line.
+
+```
+$ cd ports/nrf
+$ ./drivers/bluetooth/download_ble_stack.sh
+```
+
+## Serial Bootloader
+
+The Adafruit nRF52840 Feather uses a serial bootloader that allows you to
+update the core CircuitPython firmware and internal file system contents
+using only a serial connection.
+
+On empty devices, the serial bootloader will need to be flashed once using a
+HW debugger such as a Segger J-Link before the serial updater (`nrfutil`) can
+be used.
+
+### Install the Bootloader with `nrfjprog`
+
+```
+make SD=s140 BOARD=feather52840 boot-flash
+```
+
+## Building and Flashing CircuitPython
+
+```
+make SD=s140 SERIAL=/dev/ttyACM0 BOARD=feather52840 dfu-gen dfu-flash
+```