Update ubuntu documentation to add linux headers as well

If you are running Ubuntu with a native root zfs file system(1), following the ubuntu getting started documentation will render your system unbootable. If you only install the kernel then the zfs DKMS module never gets build and added to the initramfs so you fail to boot. The solution it so also install the kernel headers. 


1) https://github.com/zfsonlinux/pkg-zfs/wiki/HOWTO-install-Ubuntu-to-a-Native-ZFS-Root-Filesystem