README.rst: add debian+ubuntu

Reword the instructions.
Add Debian+Ubuntu instructions.
Also, the EC2 RHEL6.4 workaround described in 4413d0e6d21c648a545137baf53add55f86dc89d was no good - the sed command should have modified /etc/rc.d/rc.local (/etc/rc.local is a symlink.) Now a working workaround is in the setup.sh script...