update documentation to reflect changes for and release of 0.4.0 version

docs/upgrading.md

 # Upgrading OpenAppStack
 
-## From versions earlier than 0.3.0
+## Upgrading from 0.3.0 to 0.4.0
+
+There is no easy upgrade path from version 0.3.0 to version 0.4.0. As far as we
+know, nobody was running OpenAppStack apart from the developers, so we assume
+this is not a problem.
+
+If you do need to upgrade, this is how you can migrate your data. Backup all the
+data available under `/var/lib/OpenAppStack/local-storage`, create a new cluster
+using the installation instructions, and putting back the data. This migration
+procedure might not work perfectly.
+
+Use `kubectl get pvc -A` on your old cluster to get a mapping of all the PVC
+uuids (and thus their folder names in `/var/lib/OpenAppStack/local-storage`) to
+the pods they are bound to.
+
+Then, delete your old OpenAppStack, and install a new one with version number
+0.4.0 or higher. You can upload your backed up data into
+`/var/lib/OpenAppStack/local-storage`. All PVCs will have new unique IDs (and
+thus different folder names). You have to manually match the folders from your
+backup with the new folders.
+
+Additionally, if you want to re-use your old `settings.yaml` file, this data
+needs to be added to it:
+
+```
+prometheus_enable_ingress: false
+
+backup:
+  s3:
+    # Disabled by default. To enable, change to `true` and configure the
+    # settings below. You'll also want to add "velero" to the enabled
+    # applications a bit further in this file.
+    # Finally, you'll also need to provide access credentials as
+    # secrets; see the documentation:
+    # https://docs.openappstack.net/en/latest/installation_instructions.html#step-2-optional-cluster-backups-using-velero
+    enabled: false
+    # URL of S3 service. Please use the principal domain name here, without the
+    # bucket name.
+    url: "https://store.greenhost.net"
+    # Region of S3 service that's used for backups.
+    # For some on-premise providers this may be irrelevant, but the S3
+    # apparently requires it at some point.
+    region: "ceph"
+    # Name of the S3 bucket that backups will be stored in.
+    # This has to exist already: Velero will not create it for you.
+    bucket: "openappstack-backup"
+    # Prefix that's added to backup filenames.
+    prefix: "test-instance"
+
+# A whitelist of applications that will be enabled.
+enabled_applications:
+  # System components, necessary for the system to function.
+  - 'cert-manager'
+  - 'letsencrypt-production'
+  - 'letsencrypt-staging'
+  - 'ingress'
+  - 'local-path-provisioner'
+  - 'single-sign-on'
+  # The backup system Velero is disabled by default, see settings under `backup` above.
+  # - 'velero'
+  # Applications.
+  - 'grafana'
+  - 'loki-stack'
+  - 'nextcloud'
+  - 'prometheus'
+  - 'rocketchat'
+  - 'wordpress'
+```
+
+## Upgrading to 0.3.0
 
 Upgrading from versions earlier than `0.3.0` requires manual
 intervention.