Update installation documentation 

http://projects.bigcat.unimaas.nl/pathvisiorpc/documentation/installing-xmlrpc-library-in-r/

The repo source is no longer valid: http://www.omegahat.org/R

Not sure of an easy replacement. One can manually install via https://launchpad.net/ubuntu/+archive/primary/+files/r-omegahat-xmlrpc_0.3-0.orig.tar.gz
