Breaking update to store shard statefulsets

[craigfurman]

Store shards deployed before https://github.com/thanos-io/kube-thanos/pull/199 was merged cannot easily be updated with the latest revision of kube-thanos, because of 2 changes made to StatefulSet.spec: volumeClaimTemplates, and selector. The error I'm seeing from Kubernetes is updates to statefulset spec for fields other than 'replicas', 'template', and 'updateStrategy' are forbidden.

This isn't a huge problem because of being able to jsonnet-patch around this, but was still a little time consuming to diagnose and fix, and I wonder if there's a reasonable place to document this to save future users the time.

[metalmatze]

This is probably best to be put in the CHANGELOG.md and then to be announced by the time we publish a new release.