Left the stage overview as it was since it's intended more as a plain
language explanation than a technically complete look. I believe that
kind of information is useful to have, but perhaps as README.md becomes
longer it may be advisable to move it to another location.
Decided that the Stages heading probably made more sense as being
retitled to refer specifically to Raspbian since it has no direct
bearing on pi-gen itself which could produce any Debianish system for
the pi, and also that there ought to be a Stage Anatomy section first
talking about the nuts and bolts of what goes into these stages.
Stage Anatomy is just a heading for now.
