feat(alert): adding sample prometheus rules for ZFSPV (#32)

Provide sample instructions on setting up prometheus via prometheus-operator and then configuring a sample rule to monitor the volume space utilization, and once available space is less than 10%, it will start firing the alert.

```
 100 * kubelet_volume_stats_available_bytes{job="kubelet"}
          /
        kubelet_volume_stats_capacity_bytes{job="kubelet"}
          < 10
```

Signed-off-by: Pawan <pawan@mayadata.io>

deploy/sample/prometheus-alert.rules

@@ -0,0 +1,27 @@
+apiVersion: monitoring.coreos.com/v1
+kind: PrometheusRule
+metadata:
+  labels:
+    app: prometheus-operator
+    chart: prometheus-operator-8.5.4
+    heritage: Tiller
+    release: prometheus-operator
+  name: prometheus-operator-zfs-alertmanager.rules
+  namespace: default
+spec:
+  groups:
+  - name: zfsalertmanager.rules
+    rules:
+    - alert: ZFSVolumeUsageCritical
+      annotations:
+        message: The PersistentVolume claimed by {{ $labels.persistentvolumeclaim
+          }} in Namespace {{ $labels.namespace }} is only {{ printf "%0.2f" $value
+          }}% free.
+      expr: |
+        100 * kubelet_volume_stats_available_bytes{job="kubelet"}
+          /
+        kubelet_volume_stats_capacity_bytes{job="kubelet"}
+          < 10
+      for: 1m
+      labels:
+        severity: critical