docs: describe supported Kubernetes versions

docs/deployment/index.md

@@ -6,6 +6,9 @@ sort: 2
 
 # Deployment
 
+Node Feature Discovery can be deployed on any recent version of Kubernetes
+(v1.21+).
+
 See [Image variants](image-variants.md) for description of the different NFD
 container images available.
 

docs/get-started/introduction.md

@@ -18,7 +18,8 @@ sort: 1
 This software enables node feature discovery for Kubernetes. It detects
 hardware features available on each node in a Kubernetes cluster, and
 advertises those features using node labels and optionally node extended
-resources and node taints.
+resources and node taints. Node Feature Discovery is compatible with any recent
+version of Kubernetes (v1.21+).
 
 NFD consists of four software components:
 

docs/usage/nfd-topology-updater.md

@@ -36,8 +36,10 @@ given a configuration of resources to exclude via [`-excludeList`](../reference/
 
 ## Deployment Notes
 
-Kubelet [PodResource API][podresource-api] is a prerequisite for
-nfd-topology-updater to be able to run.
+Kubelet [PodResource API][podresource-api] with the
+[GetAllocatableResources][getallocatableresources] functionality enabled is a
+prerequisite for nfd-topology-updater to be able to run (i.e. Kubernetes v1.21
+or later is required).
 
 Preceding Kubernetes v1.23, the `kubelet` must be started with
 `--feature-gates=KubeletPodResourcesGetAllocatable=true`.
@@ -83,3 +85,4 @@ for creating a configuration.
 <!-- Links -->
 [podresource-api]: https://kubernetes.io/docs/concepts/extend-kubernetes/compute-storage-net/device-plugins/#monitoring-device-plugin-resources
 [feature-gate]: https://kubernetes.io/docs/reference/command-line-tools-reference/feature-gates
+[getallocatableresources]: https://kubernetes.io/docs/concepts/extend-kubernetes/compute-storage-net/device-plugins/#grpc-endpoint-getallocatableresources