Add comment about openedx_releases hash

vagrant/release/devstack/Vagrantfile

@@ -54,6 +54,9 @@ if ENV['VAGRANT_MOUNT_BASE']
 
 end
 
+# map the name of the git branch that we use for a release
+# to a name and a file path, which are used for retrieving
+# a Vagrant box from the internet.
 openedx_releases = {
   "openedx/rc/aspen-2014-09-10" => {
     :name => "aspen-rc1", :file => "20141009-aspen-devstack-rc1.box",