Add documentation for containerRegisty and containerProxy spec

docs/cluster_spec.md

@@ -539,3 +539,37 @@ spec:
       providerExtraConfig:
         alias: foo
 ```
+
+### assets
+
+Assets define alernative locations from where to retrieve static files and containers
+
+#### containerRegistry
+
+The container registry enables kops / kubernets to pull containers from a managed registry.
+This is useful when pulling containers from the internet is not an option, eg. because the
+deployment is offline / internet restricted or because of special requirements that apply
+for deployed artifacts, eg. auditing of containers.
+
+For a use case example, see [How to use kops in AWS China Region](https://github.com/kubernetes/kops/blob/master/docs/aws-china.md)
+
+```yaml
+spec:
+  assets:
+    containerRegistry: example.com/registry
+```
+
+
+#### containerProxy
+
+The container proxy is designed to acts as a [pull through cache](https://docs.docker.com/registry/recipes/mirror/) for docker container assets.
+Basically, what it does is it remaps the Kubernets image URL to point to you cache so that the docker daemon will pull the image from that location.
+If, for example, the containerProxy is set to `proxy.example.com`, the image `k8s.gcr.io/kube-apiserver` will be pulled from `proxy.example.com/kube-apiserver` instead.
+Note that the proxy you use has to support this feature for private registries.
+
+
+```yaml
+spec:
+  assets:
+    containerProxy: proxy.example.com
+```