Update documentation for supported versions of Helm and Kubernetes
Summary
The documentation currently reads:
1. kubectl 1.8 or higher, compatible with your cluster
([+/- 1 minor release from your cluster](https://kubernetes.io/docs/tasks/tools/install-kubectl/#before-you-begin)).
1. Helm 2.9 or higher.
1. A Kubernetes cluster, version 1.8 or higher. 6vCPU and 16GB of RAM is recommended.
(doc/installation/index.md)
The k8s version should be updated to those receiving security updates (1.10+). The Helm to maintained versions (2.10+).
cc @twk3