Mention 'KubeletPluginsWatcher' feature

This feature is more developers oriented than users oriented, so simply
mention it in the feature gate should be fine.
In future, when the design doc is migrated from Google doc to the
kubernetes/community repo, we can add links to it for users who want to
dig deeper.

Closes: kubernetes#9108
Xref: kubernetes/kubernetes#63328, kubernetes/kubernetes#64605

content/en/docs/reference/command-line-tools-reference/feature-gates.md

@@ -66,6 +66,7 @@ different Kubernetes components.
 | `HyperVContainer` | `false` | Alpha | 1.10 | |
 | `Initializers` | `false` | Alpha | 1.7 | |
 | `KubeletConfigFile` | `false` | Alpha | 1.8 | 1.9 |
+| `KubeletPluginsWatcher` | `false` | Alpha | 1.11 | |
 | `LocalStorageCapacityIsolation` | `false` | Alpha | 1.7 | 1.9 |
 | `LocalStorageCapacityIsolation` | `true` | Beta| 1.10 | |
 | `MountContainers` | `false` | Alpha | 1.9 | |
@@ -188,6 +189,8 @@ Each feature gate is designed for enabling/disabling a specific feature:
   When the `Initializers` admission controller is enabled, this feature is automatically enabled.
 - `KubeletConfigFile`: Enable loading kubelet configuration from a file specified using a config file.
   See [setting kubelet parameters via a config file](/docs/tasks/administer-cluster/kubelet-config-file/) for more details.
+- `KubletPluginsWatcher`: Enable probe based plugin watcher unility for kubelet
+  to discover plugins such as [CSI volume drivers](/docs/concepts/storage/volumes/#csi).
 - `LocalStorageCapacityIsolation`: Enable the consumption of [local ephemeral storage](/docs/concepts/configuration/manage-compute-resources-container/) and also the `sizeLimit` property of an [emptyDir volume](/docs/concepts/storage/volumes/#emptydir).
 - `MountContainers`: Enable using utility containers on host as the volume mounter.
 - `MountPropagation`: Enable sharing volume mounted by one container to other containers or pods.