Adding Vagrant option to README

We've cleaned up the Vagrant implementation of DuckPAN a bit - this should now be ready for others to use.

No issues have been reported in the [forums](https://duck.co/forum/thread/4775/duckpan-with-vagrant) and this implementation is working well for us.

I've added a description of how to get up and running with the Vagrant-based environment to the README + a brief primer on the main Vagrant commands.  This addition also checks two items off of the DuckPAN roadmap, so I've updated that, as well.