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.

.project

.directory
.pytest_cache
.cache
.coverage_full

syntax: regexp
^tests/.*/build$
^.idea/.*
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^.*\.pyc$
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^.*~$
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^.*\.swp$

bug_report.*\.txt
i18n/.*.new$
revision

doc/_build
doc/locale