Update zfs-install notes

Many improvements:

	* Provide download link to recommended live media
	* Added new portage build directory
	* Fixed regression in downloading stage3
	* Faster download of initial tree
	* Provided explicit commands for partitioning
	* Simplified cache coherence workaround
	* Provided explicit command to update fstab

Signed-off-by: Richard Yao <ryao@gentoo.org>