Bulk out README introduction.
Vagrant nodes with different IPs and RAM

Demonstrate assigning private network interfaces to VMs, in order for them
to communicate with each other. And assigning a different amount of RAM to a
single node. Nodes are defined in a Ruby hash.

VirtualBox provider customisations have been moved into the `nodes.each`
loop because it needs access to `node_opts[:memory]`. There seems to be no
speed impact to `vagrant status`. Whereas calling `customize` twice would
have been significantly slower, even if it did look cleaner.
Vagrant set node name in VirtualBox

This makes the VMs have huamn readable names in `VBoxManage` and the VBox
GUI. Which can be handy, once you've accumulated a few.