DOC: section for troubleshooting and offline installation in setup readme

This is an update of the documentation as proposed in #142 (closed) .

I suggest to check and review:

• Please test whether the setup described in "Installation without or with restricted internet connection on the host system" actually works. I'm a bit sceptical that it solves all maven related issues. Can we maybe create a unit test for that?
• Is it ok to have external sources like stackoverflow, etc. in that document?
• I have never tried the CAP_NET_BIND_SERVICE setup. Has anyone? Is it worth exploring that option? Should it be mentioned there?
• Can we add a description for the iptables setup? (Maybe open a new issue for that.)