Add a VirtualBox support matrix.

<optak> rkratky: hello, it looks that vagrant-1.7.4 can work only with VirtualBox 5.0 (or 4.0, 4.1, 4.2, 4.3). Is this documented? I installed latest VBox: 5.1.4r110228 which broke CDK

Denis Golovin: Vagrant 1.8.1 cannot detect latest VirtualBox 5.1.X installed during vagrant up and starts download for VirtualBox 5.0.10 in order to proceed. If you let it finish it will downgrade installed VirtualBox 5.1.X to 5.0.10 and continue with 'vagrant up'. Should we mention that somewhere in release notes?