docs(longhorn): Add initial README

infrastructure/longhorn/README.md

+Longhorn
+===
+
+This component is deployed to provide persistent storage replicated across the nodes in an easy fashion.
+
+Links
+---
+
+- [Docs](https://longhorn.io/docs/)
+- [Helm Chart](https://artifacthub.io/packages/helm/longhorn/longhorn)
+- [Source Code](https://github.com/longhorn)
+- [Help Forum](https://github.com/longhorn/longhorn/discussions)