website: document the guest additinos URL and SHA256

/cc @sgirones

website/source/docs/builders/virtualbox.html.markdown

@@ -76,6 +76,16 @@ Optional:
   of the user. This is a [configuration template](/docs/templates/configuration-templates.html)
   where the `Version` variable is replaced with the VirtualBox version.
 
+* `guest_additions_sha256` (string) - The SHA256 checksum of the guest
+  additions ISO that will be uploaded to the guest VM. By default the
+  checksums will be downloaded from the VirtualBox website, so this only
+  needs to be set if you want to be explicit about the checksum.
+
+* `guest_additions_url` (string) - The URL to the guest additions ISO
+  to upload. This can also be a file URL if the ISO is at a local path.
+  By default the VirtualBox builder will go and download the proper
+  guest additions ISO from the internet.
+
 * `guest_os_type` (string) - The guest OS type being installed. By default
   this is "other", but you can get _dramatic_ performance improvements by
   setting this to the proper value. To view all available values for this