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.

                  <xsd:element name="Win32">
                    <xsd:complexType>
                      %(toolchain_gcc)s
                    </xsd:complexType>
                  </xsd:element>