Doc updates for controller namespace

Make all namespace settings explicit and add more documentation around
setting appropriate namespace when installing.

deploy/kubernetes/snapshot-controller/setup-snapshot-controller.yaml

@@ -1,10 +1,16 @@
 # This YAML file shows how to deploy the snapshot controller
 
+# The snapshot controller implements the control loop for CSI snapshot functionality.
+# It should be installed as part of the base Kubernetes distribution in an appropriate
+# namespace for components implementing base system functionality. For installing with
+# Vanilla Kubernetes, kube-system makes sense for the namespace.
+
 ---
 kind: StatefulSet
 apiVersion: apps/v1
 metadata:
   name: snapshot-controller
+  namespace: default # TODO: replace with the namespace you want for your controller, e.g. kube-system
 spec:
   serviceName: "snapshot-controller"
   replicas: 1