Doc: Resizing Boot2Docker Volume with VirtualBox

The Boot2Docker VM ships with a 20G VMDK volume, which can quickly fill up if one is using large images/large numbers of images. This is a step-by-step guide for increasing the B2D volume using VirtualBox and GParted.
Agreed, we should use a proper em dash instead of double hyphens. And sure, the sentence could be broken into two.

Don't know about ISO vs. app. Probably doesn't really make much difference one way or the other. Both seem accurate.