Adds procedure about kubedb snapshots.

operators/kubedb.md

+Clean snapshots older than 2 months:
+
+```
+kubectl get snapshots.kubedb.com -o go-template --template '{{range .items}}{{.metadata.name}} {{.metadata.creationTimestamp}}{{"\n"}}{{end}}' | awk '$2 <= "'$(date -d '2 month ago' -Ins --utc | sed 's/+0000/Z/')'" { print $1 }'  | xargs --no-run-if-empty kubectl delete snapshots.kubedb.com
+```
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