Update hyper-v.md (docker#2121)

* Update hyper-v.md

* Update hyper-v.md

* Update hyper-v.md

machine/drivers/hyper-v.md

@@ -57,7 +57,9 @@ Hyper-V](https://msdn.microsoft.com/en-us/virtualization/hyperv_on_windows/quick
 
   Hyper-V is automatically enabled on a Docker for Windows install. To enable it manually, see [instructions on how to manually enable Hyper-V](https://msdn.microsoft.com/en-us/virtualization/hyperv_on_windows/quick_start/walkthrough_install) on the Microsoft developer network.
 
-#### 2. Set up a new external network switch
+#### 2. Set up a new external network switch (Optional)
+
+> **Note**: If you already have a external network switch you can skip this setup and use that one.
 
 Make sure you have Ethernet connectivity while you are doing this.