Document port and node restriction guidelines

[displague]

Include in the README guidelines about what ports are required and which ports may be restricted. Note where the API server is listening (both public and private port 6443) and offer strategies to customize this (including a NodeBalancer, once HA is included).

Terraform configuration options may be able to manage these restrictions since Terraform "knows" all of the nodes and can manage a whitelist nodes upon addition and removal. There is also the possibility to manage this within the global Network Policy using the CNI driver.

https://rancher.com/docs/rancher/v2.x/en/installation/references/ is a useful reference in terms of expectations for the depth of documentation needed. Rancher nodes and the control-plane are configured very differently so the actual ports in that document are not what would be recommended for terraform-linode-k8s.