Added contact information and updated required Vagrant version
Added contact information and updated required Vagrant version

I updated the contact information in the README.md file so users can
easily find where to get support for issues that they encounter
as well as updated the required Vagrant version according to the
documentation located here:
https://coreos.com/os/docs/latest/booting-on-vagrant.html

quantum/coreos-vagrant,j0rdn/coreos-rancher-vagrant,andrewrothstein/coreos-vagrant