ramdisk resize and image resize

Add some docs about resizing partitions on root with ramdisk

Give an example of how to use virt-resize.

Fix minor typo on front page.

Backport from https://review.openstack.org/31420

Change-Id: I7afbb7093167b316e6c9f9ed89106ef4b0f76731
Merge "ramdisk resize and image resize" into stable/grizzly