Updated instructions for Windows 10 Fall Creators Update.

doc/build-windows.md

@@ -31,6 +31,10 @@ This feature is not supported in versions of Windows prior to Windows 10 or on
 Windows Server SKUs. In addition, it is available [only for 64-bit versions of
 Windows](https://msdn.microsoft.com/en-us/commandline/wsl/install_guide).
 
+For Windows 10 systems with the Fall Creators Update applied (version >= 16215.0) use the Windows Store
+to install Ubuntu. Search for "Linux" in the Windows Store and install the free "Ubuntu" application.
+Full instructions are available on the above link.
+
 To get the bash shell, you must first activate the feature in Windows.
 
 1. Turn on Developer Mode