Add logging docs

docs/maintenance.md

 # Maintaining an Openappstack cluster
 
+## Logging
+
+Logs from pods and containers can be read in different ways:
+
+* In the cluster filesystem at `/var/log/pods/` or `/var/logs/containers/`.
+* Using [kubectl logs](https://kubectl.docs.kubernetes.io/pages/container_debugging/container_logs.html).
+* Querying aggregated logs with grafana, see below.
+
+## Central log aggregation
+
+We use [promtail](https://github.com/grafana/loki/tree/master/docs/clients/promtail),
+[Loki](https://grafana.com/oss/loki/) and [grafana](https://grafana.com/) for
+easy access of aggregated logs.
+The [Loki documentation](https://github.com/grafana/loki#documentation) is a
+good starting point how this setup works, and the [Using Loki in Grafana](https://grafana.com/docs/grafana/latest/features/datasources/loki/)
+gets you started with querying your cluster logs with grafana.
+
 ## Backup
 
 Please take care to backup the following locations: