storage: Add manifest for Longhorn

I was originally going to use GlusterFS to provide persistent storage
for pods, but [Heketi][0], the component that provides the API for
the Kubernetes StorageClass, is in "deep maintenance" status and looks
to be practically dead.  I was a bit afraid to try to use it because of
that, and went looking for guidance on Reddit, which is how I discovered
Longhorn.

storage/README.md

@@ -0,0 +1,31 @@
+# Persistent Storage Using Longhorn
+
+[Longhorn] is a Kubernetes-native persistent storage system.  It is quite
+complex, but installing it is fairly straightforward.
+
+[Longhorn]: https://longhorn.io
+
+
+## Installation
+
+This is mostly based on the official [Install with Kubectl][0] instructions.
+I wanted to add node selectors to the user-deployed and system-managed
+components, to ensure that Longhorn only runs on x86_64 nodes, per the
+[Node Selector][1] instructions.
+
+```sh
+kubectl apply -f longhorn.yaml
+```
+
+
+## Ingress
+
+Expose the Longhorn UI outside the cluster:
+
+```sh
+kubectl apply -f longhorn-ingress.yaml
+```
+
+
+[0]: https://longhorn.io/docs/1.3.0/deploy/install/install-with-kubectl/
+[1]: https://longhorn.io/docs/1.3.0/advanced-resources/deploy/node-selector/