readme: document oneliner unattended installation command (#7983)

The original suggestion for an unattended install downloads the installation script to a file, then runs that file with the --unattended argument. The install.sh file would be left behind after the suggested command was run.
This change passes the --unattended argument directly into sh. So, it's a nice one-liner like the default installation script, and it doesn't leave a dangling install.sh script.