Update kubernetes-on-aws-prerequisites.md

ref #121

Author: mumoshu

Documentation/kubernetes-on-aws-prerequisites.md

@@ -7,10 +7,25 @@ If you're deploying a cluster with kube-aws:
 
 ## Deploying to an existing VPC
 
-If you're deploying a cluster to an existing VPC:
+`kube-aws` tries its best to not modify your existing AWS resources therefore it's users' responsibility to ensure existing AWS resources provided to `kube-aws` are properly configured.
 
-* Internet Gateway needs to be added to VPC before cluster can be created
+Please note that you don't need to care if you've instructed `kube-aws` to create all the AWS resources for you i.e. you've omitted `vpcId` and `routeTableId` from `cluster.yaml`.
+
+What `kube-aws` does modify are:
+
+* Adding a record set for Kubernetes API Endpoint to an existing hosted zone you've provided via the `hostedZoneId` configuration key in `cluster.yaml`
+* Adding one or more subnet(s) to an existing VPC specified by the `vpcId`
+* Associating one or more subnet(s) to an existing route table specified by the `routeTableId`
+
+See [`cluster.yaml`](https://github.com/coreos/kube-aws/blob/master/config/templates/cluster.yaml) for more details.
+
+All the other configurations for existing AWS resources must be done property by users before `kube-aws` is run.
+
+For example, if you're deploying a cluster to an existing VPC:
+
+* An internet gateway or a NAT gateway needs to be added to VPC before cluster can be created
   * Or [all the nodes will fail to launch because they can't pull docker images or ACIs required to run essential processes like fleet, hyperkube, etcd, awscli, cfn-signal, cfn-init.](https://github.com/coreos/kube-aws/issues/120)
+* Existing route tables must have an route to Internet in some form. For example, a default route to an internet gateway or to a NAT gateway via `0.0.0.0/0` would be needed or your cluster won't come up. See [a relevant issue about it](https://github.com/coreos/kube-aws/issues/121#issuecomment-266255407).
 * Existing route tables to be reused by kube-aws must be tagged with the key `KubernetesCluster` and your cluster's name for the value.
   * Or [Kubernetes will fail to create ELBs correspond to Kubernetes services with `type=LoadBalancer`](https://github.com/coreos/kube-aws/issues/135)
 * ["DNS Hostnames" must be turned on before cluster can be created](https://github.com/coreos/kube-aws/issues/119)