Add VirtualBox version 6.0.x to docs.
Add VirtualBox version 6.0.x to docs.

chrisroberts/vagrant,chrisroberts/vagrant,bryson/vagrant,bryson/vagrant,marxarelli/vagrant,mitchellh/vagrant,gitebra/vagrant,gitebra/vagrant,bryson/vagrant,chrisroberts/vagrant,sni/vagrant,gitebra/vagrant,mitchellh/vagrant,mitchellh/vagrant,sni/vagrant,mitchellh/vagrant,bryson/vagrant,marxarelli/vagrant,marxarelli/vagrant,gitebra/vagrant,sni/vagrant,marxarelli/vagrant,chrisroberts/vagrant,sni/vagrant