docs: document plan to remove v1alpha3 and v1alpha4 apiVersions

Signed-off-by: Stefan Büringer buringerst@vmware.com

CONTRIBUTING.md

@@ -161,6 +161,20 @@ Cluster API maintains the most recent release/releases for all supported API and
 
 - Exceptions can be filed with maintainers and taken into consideration on a case-by-case basis.
 
+### Removal of v1alpha3 & v1alpha4 apiVersions
+
+The last releases supporting v1alpha3 (v0.3.x) and v1alpha4 (v0.4.x) apiVersions have been both EOL since April 2022.
+The apiVersions have been kept around to allow upgrades from v0.3.x and v0.4.x Cluster API versions to v1.x. Given that 
+these releases have been EOL for a significant period, we are going to remove the apiVersions in upcoming releases:
+* v1.5:
+  * Kubernetes API server will stop serving the v1alpha3 apiVersion
+* v1.6:
+  * v1alpha3 apiVersion will be removed from the CRDs
+  * Kubernetes API server will stop serving the v1alpha4 apiVersion
+* v1.7
+  * v1alpha4 apiVersion will be removed from the CRDs
+For more details and latest information please see the following issue: [Removing v1alpha3 & v1alpha4 apiVersions](https://github.com/kubernetes-sigs/cluster-api/issues/8038).
+
 ## Contributing a Patch
 
 1. If you haven't already done so, sign a Contributor License Agreement (see details above).