Add comment about load path and VAGRANT_VAGRANTFILE

There seems to be a bug with VAGRANT_VAGRANTFILE (or perhaps it's
expected behavior) where the load path changes and it doesn't get
configuration items for the Centos box's Vagrantfile (which is
located in ~/.vagrant.d/boxes/Centos/Vagrantfile).  Because of that,
the configuration item would need to be set in the Vagrantfile specified
in VAGRANT_VAGRANTFILE, OR you can just rename the Vagrantfile in
question to be "Vagrantfile" and Robert is your father's brother...