Add README for updating Nextion displays by python script

Nextion/README.md

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+# Update Nextion Displays from the Command Line
+
+This directory contains a simple python script which you can use to update the
+Nextion displays. All you need is a compiled .tft file which is written to the
+display's flash memory. The precompiled .tft files with the MMDVMHost default
+layout are to be found in this directory as well.
+
+To update the Nextion display you just need to know the serial port the display
+is connected to. It could be /dev/ttyUSBx for USB<->Serial adapters or
+/dev/ttyAMA0 for the UART on the Raspberry Pi for example.
+
+# Prerequisites
+
+You need to have python installed as well as the python-serial package. That can
+normally be found in your distro's package manager.
+
+# Updating the display
+
+Now comes the easy part. Just execute:
+
+```
+$ python nextion.py MMDVM_3.5.tft /dev/ttyUSB0
+```
+
+And the output should be as follows:
+
+```
+Trying with baudrate: 2400...
+Trying with baudrate: 4800...
+Trying with baudrate: 9600...
+Connected with baudrate: 9600...
+Status: comok
+Touchscreen: yes
+Model: NX4832T035_011R
+Firmware version: 68
+MCU code: 61488
+Serial: D2658880C35D2124
+Flash size: 16777216
+Downloading, 100%...
+File transferred successfully
+```
+
+# Known errors
+
+Especially when using USB<->Serial adapters there are cases in which the scripts
+stops at different times. This is known and due to the very simple update
+protocol. In this case you have to fix the display by using a SD-Card to update
+the display. The /dev/ttyAMAx ports do not seems to suffer from this issue.
+