Update docs with instructions for the new AWS CNI to allow connecting to external service

Setup based on docs doesn't work on a fresh cluster with AWS CNI above version 1.12.6 as it doesn't allow routing services to endpoints outside the cluster. At first I switched to Calico which seemed to fix the issue, but there is a simpler way:
```
kubectl set env daemonset aws-node -n kube-system "AWS_EXTERNAL_SERVICE_CIDRS=/16"
```
This isn't strictly cloud map issue, but I believe docs will benefit from including this as currently the steps can't be followed exactly. I'm willing to submit a PR adding section to the docs if there is approval, though I think someone more versed might be better suited as I haven't explored alternative solutions.

Hope this helps someone ✌️

`AWS_EXTERNAL_SERVICE_CIDRS` was introduced in: https://github.com/aws/amazon-vpc-cni-k8s/pull/2243

Read the existing setup documentation for connecting to external services on a fresh cluster, then verify the AWS CNI version requirement and the AWS_EXTERNAL_SERVICE_CIDRS kubectl command described in the issue. Done means the documented steps work with AWS CNI above 1.12.6 and clearly explain the additional configuration.

Written by the indexing model from the issue text.