Update -DomainMode for Windows Server 2016 Domain

Hi, this is important!

Author: PhilipElder

docset/windows/addsdeployment/install-addsdomain.md

@@ -173,6 +173,7 @@ The acceptable values for this parameter are:
 - Windows Server 2008 R2:  4 or Win2008R2
 - Windows Server 2012:  5 or Win2012
 - Windows Server 2012 R2:  6 or Win2012R2
+- Windows Server 2016: 7 or Windows2016Domain
 
 The domain functional level cannot be lower than the forest functional level, but it can be higher.
 The default is automatically computed and set.