Add docs about vagrant box

Add a section briefly describing the vagrant box and its included functionality.  I am adding it here as well as the community-plugins repo as I see this as more of a how-to-develop repo and feel that documentation concerning developing/testing plugins should be stored here vs the community-plugins repo which is more of an end-user repo.

The community-plugins repos only has a brief paragraph about the box and then points the user to here for details.