Added missing HPAScaleToZero feature gate (#16592)

I discovered this feature gate when trying to configure scale-to-zero but found that it wasn't included in the documentation.

Author: antoinne85

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

@@ -91,6 +91,7 @@ different Kubernetes components.
 | `ExpandPersistentVolumes` | `true` | Beta | 1.11 | |
 | `ExperimentalHostUserNamespaceDefaulting` | `false` | Beta | 1.5 | |
 | `EvenPodsSpread` | `false` | Alpha | 1.16 | |
+| `HPAScaleToZero` | `false` | Alpha | 1.16 | |
 | `HyperVContainer` | `false` | Alpha | 1.10 | |
 | `KubeletPodResources` | `false` | Alpha | 1.13 | 1.14 |
 | `KubeletPodResources` | `true` | Beta | 1.15 | |
@@ -347,6 +348,7 @@ Each feature gate is designed for enabling/disabling a specific feature:
 - `GCERegionalPersistentDisk`: Enable the regional PD feature on GCE.
 - `HugePages`: Enable the allocation and consumption of pre-allocated [huge pages](/docs/tasks/manage-hugepages/scheduling-hugepages/).
 - `HyperVContainer`: Enable [Hyper-V isolation](https://docs.microsoft.com/en-us/virtualization/windowscontainers/manage-containers/hyperv-container) for Windows containers.
+- `HPAScaleToZero`: Enables setting `minReplicas` to 0 for `HorizontalPodAutoscaler` resources when using custom or external metrics.
 - `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.
 - `KubeletPluginsWatcher`: Enable probe-based plugin watcher utility to enable kubelet