Merge pull request #2157 from infosiftr/windows-dind

Add specific Windows note to Docker about the image being client-only

docker/variant-windowsservercore.md

@@ -0,0 +1,3 @@
+## `%%IMAGE%%:<version>-windowsservercore`
+
+Unfortunately, Windows does not support nested containers, so this image variant only contains the client (intended for use against an existing Docker engine, ala `-v //./pipe/docker_engine://./pipe/docker_engine`).