Improve documentention for Arch Linux and change default listening port

[berturion]

I just wanted you to know that in Arch Linux you must install python-flask from community repo and python-flask-cors from AUR in order to be able to run xhu.py with CORS enabled (these packages provide the necessary dependencies).

Also, the default port used by flask seems to be 5000. This is a problem when mod_proxy65 is enabled in prosody and use the default port because it is the same and your flask program will refuse to listen to port 5000 as it is already occupied by prosody.

So one would change the ExecStart command, providing a port with --port=5050 in systemd unit, to:

ExecStart=/usr/bin/flask run --port=5050

And then replace 5000 by 5050 in the Nginx reverse proxy config.