Added building comment

git-svn-id: https://redmine.imt-atlantique.fr/svn/xaal/code/Python/trunk@1996 b32b6428-25c9-4566-ad07-03861ab6144f

devices/protocols/knx/README.rst

@@ -25,6 +25,10 @@ run :
 If you use an USB (serial) port, you can use knxd to forward commands
 between serial & multicast. 
 
+to build knxd : 
+git clone https://github.com/knxd/knxd.git
+./configure --enable-usb  --prefix=/opt/knxd --disable-systemd
+
 sudo knxd -t 1023 -e 0.0.1 -E 0.0.2:8 -i -R -S -b usb: