Add M5 boards compatibility and low power mode (#586)

docs/prerequisites/M5.md

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+# M5 Boards
+M5 boards, manufactured by [M5Stack](https://m5stack.com/) are inexpensive, well featured boards based on ESP32.
+
+## M5StickC and M5Stack
+M5StickC and M5Stack are supported by OpenMQTTGateway and provide the following features:
+
+### 3 low power modes 
+#### Always ON display
+The display is always ON but decrease its brightness when not processing.
+
+#### ON display when processing
+The display is OFF when not processing and the ESP32 is put in low power mode.
+
+#### Always OFF display
+The display is always OFF, if the board has a led, the led is used when processing.
+
+To change the low power mode you can do it by [MQTT](../use/boards#low-power-mode-for-m5-boards) or by pushing the middle button (btn B) of the M5Stack and the top button (btn B) of the M5Stick C
+
+### Start screen
+![](../img/OpenMQTTgateway_M5_Stack_Board_Display_Logo.png)
+
+### Sleep screen
+The screen brightness is put to a minimum so as to reduce power consumption.
+![](../img/OpenMQTTgateway_M5_Stack_Board_Display_Text.png)
+
+### Log display in case of warning
+If the gateway detects a warning or an error the sleep go out the sleep screen and show the message with a red (error) or orange (warning) background.
+![](../img/OpenMQTTgateway_M5_Stack_Board_Display_Warning.png)
+
+The M5Stick support low power mode 2 but its display is not supported for the moment.