deploy: add README, fix typos

The csi-hostpath-snapshotter.yaml was misspelled when creating the
file. The upstream driver-registrar RBAC file uses
"csi-driver-registrar" as service account name and therefore the
hostpath plugin should also use that instead of the (now obsolete)
"csi-node-sa" name from the Kubernetes E2E testing.

This dependency on the upstream RBAC rules should be mentioned. For
the sake of avoiding yet another copy of URLs the new README just
refers to other documentation.

deploy/hostpath/README.md

@@ -0,0 +1,6 @@
+The .yaml files in this directory depend on the RBAC files that were
+released together the CSI sidecar containers. Check the repos and/or
+release notes for the CSI sidecar containers for details.
+
+https://kubernetes-csi.github.io/docs/ has an example that puts all
+pieces together.

deploy/hostpath/csi-hostpathplugin.yaml

@@ -11,7 +11,7 @@ spec:
       labels:
         app: csi-hostpathplugin
     spec:
-      serviceAccountName: csi-node-sa
+      serviceAccountName: csi-driver-registrar
       hostNetwork: true
       containers:
         - name: driver-registrar