Add support for flex volume. Flex volume adds support for thirdparty(vendor)

volumes and custom mounts.

docs/user-guide/volumes.md

@@ -66,6 +66,7 @@ Familiarity with [pods](pods.md) is suggested.
     - [secret](#secret)
     - [persistentVolumeClaim](#persistentvolumeclaim)
     - [downwardAPI](#downwardapi)
+    - [FlexVolume](#flexvolume)
   - [Resources](#resources)
 
 <!-- END MUNGE: GENERATED_TOC -->
@@ -420,6 +421,14 @@ It mounts a directory and writes the requested data in plain text files.
 
 See the [`downwardAPI` volume example](downward-api/volume/README.md)  for more details.
 
+### FlexVolume
+
+A `FlexVolume` enables users to mount vendor volumes into a pod. It expects vendor
+drivers are installed in the volume plugin path on each kubelet node. This is
+an alpha feature and may change in future.
+
+More details are in [here](../../examples/flexvolume/README.md)
+
 ## Resources
 
 The storage media (Disk, SSD, etc) of an `emptyDir` volume is determined by the