Update build instruction for BACNet in README

in r3189 Steve added new feature to bacnet-stack: network port - to
access configuration and properties of network ports of a device.
https://sourceforge.net/p/bacnet/code/3189/

It's not implemented by Beremiz.

# -*- coding: utf-8 -*-

# See COPYING.Runtime file for copyrights details.


def _(x):
    """No translation"""
    return x


Broken = _("Broken")
Started = _("Started")
Stopped = _("Stopped")
Disconnected = _("Disconnected")
Empty = _("Empty")