Add local volume documentation (#4050)

* Add local volume documentation

* Add PV local volume example

docs/concepts/storage/volumes.md

@@ -91,6 +91,7 @@ Kubernetes supports several types of Volumes:
    * `PortworxVolume`
    * `ScaleIO`
    * `StorageOS`
+   * `local`
 
 We welcome additional contributions.
 
@@ -738,6 +739,55 @@ spec:
 
 For more information including Dynamic Provisioning and Persistent Volume Claims, please see the [StorageOS examples](https://github.com/kubernetes/kubernetes/tree/{{page.githubbranch}}/examples/volumes/storageos).
 
+### local
+
+This volume type is alpha in 1.7.
+
+A `local` volume represents a mounted local storage device such as a disk,
+partition or directory.
+
+Local volumes can only be used as a statically created PersistentVolume.
+
+Compared to HostPath volumes, local volumes can be used in a durable manner
+without manually scheduling pods to nodes, as the system is aware of the volume's
+node constraints.
+
+However, local volumes are still subject to the availability of the underlying
+node and are not suitable for all applications.
+
+The following is an example PersistentVolume spec using a `local` volume:
+
+``` yaml
+apiVersion: v1
+kind: PersistentVolume
+metadata:
+  name: example-pv
+  annotations:
+        "volume.alpha.kubernetes.io/node-affinity": '{
+            "requiredDuringSchedulingIgnoredDuringExecution": {
+                "nodeSelectorTerms": [
+                    { "matchExpressions": [
+                        { "key": "kubernetes.io/hostname",
+                          "operator": "In",
+                          "values": ["example-node"]
+                        }
+                    ]}
+                 ]}
+              }'
+spec:
+    capacity:
+      storage: 100Gi
+    accessModes:
+    - ReadWriteOnce
+    persistentVolumeReclaimPolicy: Delete
+    storageClassName: local-storage
+    local:
+      path: /mnt/disks/ssd1
+```
+
+For details on the `local` volume type, see the [Local Persistent Storage 
+user guide](https://github.com/kubernetes-incubator/external-storage/tree/master/local-volume)
+
 ## Using subPath
 
 Sometimes, it is useful to share one volume for multiple uses in a single pod. The `volumeMounts.subPath`