kubeadm-windows

Instructions

Provision a control plane node with kubeadm and grab the kubeadm join command from it
Install docker and the Windows "Containers" feature and reboot the machine, or use a VM template that already has these installed. A script to do this is available here.
Install kubelet, kubeadm and wins and ensure they are on your path and that wins and kubelet are installed as services. A script to do these steps is available here. This script also will create a docker network named host, which we need for getting kubelet to allow us to run Flannel as a DaemonSet.
Run the kubeadm join command on your Windows node.
Install flannel onto your cluster using the manifest here. This installs DaemonSets for both Linux and Windows nodes.
Install kube-proxy onto your cluster using the manifest here. This is only a Windows DaemonSet as the Linux one is installed by kubeadm.
Your nodes should become Ready as Flannel initializes and configures networking.

We've tested bringing up a Linux cluster with kind and joining a Windows VM to it from VMware Fusion. The same should work for tools like Virtualbox or Hyper-V, the only requirement is that the Windows node is on the same network as the kind cluster.
The kubeadm join command can be retrieved by docker exec-ing onto the control plane container and running kubeadm token create --print-join-command.
A minimal kind config is included here. This just removes the default CNI so we can use Flannel instead.

wins has some security implications, we need to investigate / recommend solutions for controlling access to its API.
The kubelet binary needed is right now a custom build since the patch in kubernetes/kubernetes#84649 is needed to enable hostNetworking for Windows pods and as of now has not been released in an official release.