Updated root README to explain the presence of overlay.tgz
Updated root README to explain the presence of overlay.tgz

wbthomason/rosbeam,Cornell-RPAL/rosbeam,Cornell-RPAL/rosbeam,Cornell-RPAL/rosbeam,wbthomason/rosbeam,wbthomason/rosbeam,Cornell-RPAL/rosbeam,wbthomason/rosbeam