update readme

Author: semuadmin

README.md

@@ -104,6 +104,8 @@ For more comprehensive installation instructions, please refer to [INSTALLATION.
 1. By default, the Settings panel is displayed to the right of the main application window. It can be hidden or shown via Menu..View..Hide/Show Settings. The panel can also be 'undocked' from the main application window via Menu..View..Undock Settings and - if [non-transient](#transient) (`transient_dialog_b: 0`) - minimized independently of the main window. Exiting the undocked dialog, or selecting Menu..View..Dock Settings, will 'dock' the panel.
 1. To connect to a GNSS receiver via USB or UART port, select the device from the listbox, set the appropriate serial connection parameters and click 
 ![connect icon](https://github.com/semuconsulting/PyGPSClient/blob/master/src/pygpsclient/resources/usbport-1-24.png?raw=true). The application will endeavour to pre-select a recognised GNSS/GPS device but this is platform and device dependent. Press the ![refresh](https://github.com/semuconsulting/PyGPSClient/blob/master/src/pygpsclient/resources/iconmonstr-refresh-6-16.png?raw=true) button to refresh the list of connected devices at any point. `Rate bps` (baud rate) is typically the only setting that might need adjusting, but tweaking the `timeout` setting may improve performance on certain platforms. The `Msg Mode` parameter defaults to `GET` i.e., periodic or poll response messages *from* a receiver. If you wish to parse streams of command or poll messages being sent *to* a receiver, set the `Msg Mode` to `SET` or `POLL`. An optional serial or socket stream inactivity timeout can also be set (in seconds; 0 = no timeout).
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+   If you get a permissions error on attempting to connect to a serial port e.g. `[Errno 13] permission denied /dev/ttyACM0`, refer to the [Installation Guidelines - User Privileges](https://github.com/semuconsulting/PyGPSClient/blob/master/INSTALLATION.md#user-privileges).
 1. A custom user-defined serial port can also be passed via the json configuration file setting `"userport_s":`, via environment variable `PYGPSCLIENT_USERPORT` or as a command line argument `--userport`. A special userport value of "ubxsimulator" invokes the experimental [`pyubxutils.UBXSimulator`](https://github.com/semuconsulting/pyubxutils/blob/main/src/pyubxutils/ubxsimulator.py) utility to emulate a GNSS NMEA/UBX serial stream. 
 1. To connect to a TCP or UDP socket, enter the server URL and port, select the protocol (defaults to TCP) and click 
 ![connect socket icon](https://github.com/semuconsulting/PyGPSClient/blob/master/src/pygpsclient/resources/ethernet-1-24.png?raw=true). For encrypted TLS connections, tick the 'TLS' checkbox. Tick the 'Self Sign' checkbox to accommodate self-signed TLS certification (*typically for test or demonstration services*).