Add "variant-windowsservercore" for discussing our new "windowsservercore" variants and how to use them

.template-helpers/variant-windowsservercore.md

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+## `%%REPO%%:windowsservercore`
+
+This image is based on [Windows Server Core (`microsoft/windowsservercore`)](https://hub.docker.com/r/microsoft/windowsservercore/). As such, it only works in places which that image does, such as Windows 10 Professional/Enterprise (Anniversary Edition) or Windows Server 2016.
+
+For information about how to get Docker running on Windows, please see the relevant "Quick Start" guide provided by Microsoft:
+
+-	[Windows Server Quick Start](https://msdn.microsoft.com/en-us/virtualization/windowscontainers/quick_start/quick_start_windows_server)
+-	[Windows 10 Quick Start](https://msdn.microsoft.com/en-us/virtualization/windowscontainers/quick_start/quick_start_windows_10)