Based on info from consul-helm issue 16, the formatting of the helm
chart value for joining an external cluster needs to be specified
as a yaml array. This updates the documentation to reflect this.
* website: initial Kubernetes section with Helm information
* website: extraConfig for clients
* website: add more helm fields
* website: document extraVolumes
* website: document Consul DNS
* website: fix typos and show example of downward API