Non-root install: tell user how to check required dependencies

For the non-root install scenario, we should tell users that it is their responsibility to ensure their system has any packages installed that our package depends on. For Debian/Ubuntu, they can run

dpkg --info couchbase-server.deb | grep Depends

to see the list. For Red Hat/Centos, they can run

rpm -qp couchbase-server.rpm --requires