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.
I don't think an additional term ("spin") should be used to refer to the process of creating / running a container.
Sticking with one of the two that are already used would help to keep things simpler and clear.

Therefore:

> If you're running a lot of containers [...]