Another documentation update.
Rework handling of Vagrant VMs.

This corrects one bug and adds one feature.

The bug: Guard was not starting Vagrant VMs if vagrant up had not been run previously.

The feature: Sandboxing can now be enabled for specific VMs. Sandboxing should only be used for those VMs that need to be rolled back, e.g. VMs used specifically for testing the system under test.
Disable rb-readline gem by default.