Add information about longhorn installation on k3s

admin/README.md

@@ -99,6 +99,23 @@ helm upgrade -v 2 --install -f production/values-geosphere-minio.yaml $EXTRA_ARG
 Note, `geosphere-minio` in the above commands must match the name of the release
 from the original installation.
 
+### Longhorn - Shared Block Storage
+
+Most cloud platforms have some concept of a shared block storage (AWS EBS,
+GCP Persistent Storage, etc). These can be mounted as normal volumes in our
+containers. Although our K3S installation has a local path provisioner these
+volumes are limited to one single node. We need another solution that shares
+the volumes between nodes. That's where longhorn comes in.
+
+Follow the official longhorn installation instructions:
+
+https://longhorn.io/docs/1.0.0/deploy/install/install-with-helm/
+
+If you have a particular mount on the cluster nodes that has more space than
+the default `/var` path, you may want to customize this setting. For longhorn
+1.0 you can do this by adding `--set defaultSettings.defaultDataPath=/data` to
+your helm install command.
+
 ### Storage - Local Large Cache
 
 **DEPRECATED**: See local path provisioner above.