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.
@fredlf that's not technically correct. You can manage more than one VM using the boot2docker cli, though I think that was broken in 1.2.0 due to a mixup in the refactoring and me.