Windows needs hardware virtualization

VirtualBox needs hardware virtualization for
64-bit guests.

Docker-DCO-1.1-Signed-off-by: Robert Bachmann <rb@robertbachmann.at> (github: robertbachmann)

docs/sources/installation/windows.md

@@ -5,7 +5,7 @@ page_keywords: Docker, Docker documentation, Windows, requirements, virtualbox,
 # Windows
 > **Note:**
 > Docker has been tested on Windows 7.1 and 8; it may also run on older versions.
-
+> Your processor needs to support hardware virtualization.
 
 The Docker Engine uses Linux-specific kernel features, so to run it on Windows
 we need to use a lightweight virtual machine (vm).  You use the Windows Docker client to