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.
Move contextual info closer to start of sentence, "The VM should long in automatically, but if it doesn't, the credentials are `docker/tcuser`.