Update initial setup instructions.

This addresses a couple of issues @fofr encountered getting a
development server running.
Document the shared assets in more detail.

On a related note: *damn*, this is complicated. I think this is because
development has historically been on a combination of host machines and
VMs, so we're having to support both approaches. For now.
Merge pull request #1452 from alphagov/initial-setup-notes

Update initial setup instructions.