Update docs to append to, not replace, /etc/hosts

The previous command (sudo tee /etc/hosts) completely replaces /etc/hosts with
standard input. The -a option (sudo tee -a /etc/hosts) ensures standard input is
only appended to /etc/hosts.
Merge pull request #1515 from openjck/append-etc-hosts

Update docs to append to, not replace, /etc/hosts