Describe image customizations to support resize.

Add a section to describe how images should be customized to support
OpenStack.

In particular, it adds a section that describes how to support
resizing partitions on boot.

Patchset has minor edits (wilil for will).

Cherry-picked from https://review.openstack.org/18804

Change-Id: I890f6cce5eeb11852f3e9dbb87e16afba1862d1a