If a user installs the default Helm chart Consul on a Kubernetes
cluster that is open to the internet, it is lacking some important
security configurations.
This adds two Helm chart values into the documentation with details
that have come up in several issues.
Additionally, it notes that persistent volumes and their claims need
to be removed manually because of current kubernetes and helm design.
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
