FDB Kubernetes Operator upgrade documentation

I thought it would be a good idea to share that we are currently working on a document to better describe the upgrade mechanism in the FDB Kubernetes Operator and some potential issues that can be observed: https://github.com/FoundationDB/fdb-kubernetes-operator/pull/1511

Given the limitation on collaboration on GitHub feel free to post questions here, or if anything important is missing from your point of view.