Add initial docs on the kops side of authentication

docs/authentication.md

@@ -0,0 +1,34 @@
+# Authentication
+
+Kops has support for configuring authentication systems.  This support is
+currently highly experimental, and should not be used with kubernetes versions
+before 1.8.5 because of a serious bug with apimachinery (#55022)[https://github.com/kubernetes/kubernetes/issues/55022].
+
+## kopeio authentication
+
+If you want to experiment with kopeio authentication, you can use
+`--authentication kopeio`.  However please be aware that kopeio authentication
+has not yet been formally released, and thus there is not a lot of upstream
+documentation.
+
+Alternatively, you can add this block to your cluster:
+
+```
+authentication:
+  kopeio: {}
+```
+
+For example:
+
+```
+apiVersion: kops/v1alpha2
+kind: Cluster
+metadata:
+  name: cluster.example.com
+spec:
+  authentication:
+    kopeio: {}
+  authorization:
+    rbac: {}
+```
+