Document use_linked_clone property for VirtualBox provider.

website/docs/source/v2/virtualbox/configuration.html.md

@@ -36,6 +36,30 @@ config.vm.provider "virtualbox" do |v|
 end
 ```
 
+## Linked Clones
+
+By default new machines are created by importing the base box. For large
+boxes this produces a large overhead in terms of time (the import operation)
+and space (the new machine contains a copy of the base box's image).
+Using linked clones can drastically reduce this overhead.
+
+Linked clones are based on a master VM, which is generated by importing the
+base box only once the first time it is required. For the linked clones only 
+differencing disk images are created where the parent disk image belongs to
+the master VM.
+
+```ruby
+config.vm.provider "virtualbox" do |v|
+  v.use_linked_clone = true
+end
+```
+
+<div class="alert alert-info">
+	<strong>Note:</strong> the generated master VMs are currently not removed
+	automatically by Vagrant. This has to be done manually. However, a master
+	VM can only be remove when there are no linked clones connected to it.
+</div>
+
 ## VBoxManage Customizations
 
 [VBoxManage](http://www.virtualbox.org/manual/ch08.html) is a utility that can