Update rootfs README Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>

change the rootfs mount location and add the README Previous path was $libdir/lxc, changed to $libdir/lxc/rootfs. Added a README file to be placed in this directory, describing the purpose of this empty directory. Having a file to be installed in this directory makes the Makefile to automatically create the directory at install time. Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>