Ported documentation (#413)

* ported documentation from develop

* adapted instructions

* fix broken English

docs/boards/nrf52840_feitian.md

@@ -0,0 +1,23 @@
+# <img alt="OpenSK logo" src="../img/OpenSK.svg" width="200px">
+
+## Feitian OpenSK USB Dongle
+
+### Flashing using DFU
+
+This board is similar in hardware to the Nordic nRF52840 Dongle. You can use DFU
+to flash it, instructions to enter DFU mode depend on the version of your
+hardware. See
+[Feitian's instructions](https://feitiantech.github.io/OpenSK_USB/). In short:
+
+*   In V1, use a paperclip to press the Reset button through the tiny hole.
+*   In V2, push and hold the user button for more than 10 seconds after
+    connecting your device.
+
+Afterwards, you can flash your Feitian OpenSK using DFU following the
+[instructions for the Nordic nRF52840 Dongle](nrf52840_dongle.md#Flashing-using-DFU).
+
+### Buttons and LEDs
+
+For both hardware versions, the buttons and LEDs are described in detail in the
+[hardware section](https://feitiantech.github.io/OpenSK_USB/hardware/) of
+Feitian's website.