docs: add English version

docs/en/README.md

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+# CLEVER
+
+This is CLEVER drone kit documentation in English.

docs/en/SUMMARY.md

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+# Summary
+
+* [UART settings](uart.md)

docs/en/uart.md

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+# UART interface
+
+UART is an asynchronous serial interface for data transfer that is used in many devices. For example, GPS antennas, Wi-Fi routers, or Pixhawk.
+
+The interface usually contains two lines: TX for data transmission, and RX for data reception. It usually uses the 5-volt logics.
+
+To connect two devices, you have to feed the TX line of the first device to the RX line of the other one. A similar manipulation is required on the other end for ensuring two-way data transmission.
+
+It is also necessary to synchronize the voltages – connect the ground on two devices.
+
+Read more about the interface and the Protocol in [this article](https://habr.com/post/109395/).
+
+## Linux TTY
+
+In Linux, there is the concept of Posix Terminal Interface (read more [here](https://ru.wikipedia.org/wiki/TTY-абстракция)). It is an abstraction over the serial or virtual interface that allows several agents to work with the device simultaneously.
+
+An example of such abstraction in Raspbian may be `/dev/tty1` – the device for text output to the screen connected via HDMI.
+
+## UART on Raspberry Pi 3
+
+Raspberry Pi 3 has two hardware UART interfaces:
+
+1. Mini UART (`/dev/ttyAMA0`) – uses the timing of the RPi graphics core, and therefore limits its frequency.
+2. PL011 (`/dev/ttyS0`) – the full-fledged UART interface on a separate chip of the microcontroller.
+
+Read more about UART on Raspberry Pi in the [official article](https://www.raspberrypi.org/documentation/configuration/uart.md).
+
+Using microcontroller valves, these interfaces may be switched between two physical outputs:
+
+1. UART connector on GPIO;
+2. RPi Bluetooth module.
+
+By default, Raspberry Pi 3 PL011 is connected to the Bluetooth module. And Mini UART is disabled with the value of directive `enable_uart`, which is `0` by default.
+
+> **Note** One should understand that directive `enable_uart` changes its default value, depending on which UART is connected to the RPi Bluetooth module with directive `dtoverlay=pi3-miniuart-bt`.
+
+For the sake of convenience of working with these outputs, aliases exist in Raspbian:
+
+* `/dev/serial0` – always points to the TTY device that is connected to the GPIO ports.
+* `/dev/serial1` – always points to the TTY device that is connected to the Bluetooh module.
+
+### Configuration of UART on Raspberry Pi 3
+
+To configure UART, there are directives located in `/boot/config.txt`.
+
+To enable the UART interface on GPIO:
+
+```txt
+enable_uart=1
+```
+
+To disconnect the UART interface from the Bluetooth module:
+
+```txt
+dtoverlay=pi3-disable-bt
+```
+
+To connect Mini UART to the Bluetooth module:
+
+```txt
+dtoverlay=pi3-miniuart-bt
+```
+
+If the Bluetooth module is disabled, one should disable the hciuart service:
+
+```bash
+sudo systemctl disable hciuart.service
+```
+
+## Default image configuration
+
+In image CLEVER, we initially disabled Mini UART and the Bluetooth module.
+
+Bugs
+----
+
+If you use the Mini UART connection to Bluetooth, `hciuart` crashes with the following error:
+
+![hciuart error](assets/hciuart_error.jpg)
+
+In case of Bluetooth disconnection
+
+```txt
+/dev/serial0 -\> ttyAMA0
+/dev/serial1 -\> ttyS0
+```