document StatefulSet vs Deployment, leadership election

We probably don't have good documentation about when to use StatefulSet and when to use Deployment for the controller. See https://github.com/container-storage-interface/spec/issues/364

Leadership election needs to be covered, too.

Start by reading the linked CSI spec issue 364 and inspect the existing documentation in the repository. Document when a controller should use StatefulSet versus Deployment, and include leadership election guidance; the work is done when both topics are covered clearly for CSI integration with Kubernetes.

Written by the indexing model from the issue text.