Merge pull request #117 from m-lab/sandbox-soltesz-readme

Clarify how to enable shimcni

configs/stage3_coreos/README.md

@@ -1,4 +1,18 @@
-The `shim.sh` file exists to aid in the debugging of CNI plugins.  It will
-likely be turned on in production for a while, but eventually the kubelet on
-the node should be set up to use the files in `/usr/cni/bin` instead of
-`/usr/shimcni/bin`.
+# CoreOS image
+
+The `shim.sh` file exists to aid in the debugging of CNI plugins. It must be
+enabled manually.
+
+For example, add a new parameter `--cni-bin-dir=/usr/shimcni/bin` to the `KUBELET_KUBECONFIG_ARGS` in the `10-kubeadm.conf` systemd unit file:
+`/etc/systemd/system/kubelet.service.d/10-kubeadm.conf`
+
+```sh
+Environment="KUBELET_KUBECONFIG_ARGS=--node-labels=mlab/machine=mlab4,mlab/site=den04,mlab/metro=den,mlab/type=platform --dynamic-config-dir=/var/lib/kubelet/dynamic-configs --bootstrap-kubeconfig=/etc/kubernetes/bootstrap-kubelet.conf --kubeconfig=/etc/kubernetes/kubelet.conf"
+```
+
+Then restart the kubelet:
+
+```sh
+systemctl daemon-reload
+systemctl restart kubelet
+```