Make note of issue with centos/7 Vagrant box

Vagrantfile

@@ -13,6 +13,10 @@ Vagrant.configure(2) do |config|
    c.vm.box = "archlinux/archlinux"
  end

  # NOTE: This box uses rsync for some reason.  Have to do 'vagrant halt
  # centos' and 'vagrant up centos' to refresh /vagrant.

  # https://github.com/mitchellh/vagrant/issues/6940
  config.vm.define "centos" do |c|
    c.vm.box = "centos/7"
  end