docs: Add breaking changes section to introduction.md

We want a way to call out some upcoming breaking changes for new
versions. This adds a note style callout in the
`ClusterAPI documentation versions` section.

docs/book/src/introduction.md

@@ -4,6 +4,14 @@ Cluster API is a Kubernetes sub-project focused on providing declarative APIs an
 
 Started by the Kubernetes Special Interest Group (SIG) [Cluster Lifecycle](https://github.com/kubernetes/community/tree/master/sig-cluster-lifecycle#readme), the Cluster API project uses Kubernetes-style APIs and patterns to automate cluster lifecycle management for platform operators. The supporting infrastructure, like virtual machines, networks, load balancers, and VPCs, as well as the Kubernetes cluster configuration are all defined in the same way that application developers operate deploying and managing their workloads. This enables consistent and repeatable cluster deployments across a wide variety of infrastructure environments.
 
+## ⚠️ Breaking Changes ⚠️
+
+- API version v1alpha3 support will be removed in the upcoming release of v1.5
+- API version v1alpha4 support will be removed in the upcoming release of v1.6
+
+Review the [support-and-guarantees](./CONTRIBUTING.md#support-and-guarantees) section of the
+contributing guide for more details.
+
 ## Getting started
 
 * [Quick Start](./user/quick-start.md)
@@ -25,6 +33,8 @@ This book documents ClusterAPI v1.4. For other Cluster API versions please see t
 * [release-0-4.cluster-api.sigs.k8s.io](https://release-0-4.cluster-api.sigs.k8s.io)
 * [release-0-3.cluster-api.sigs.k8s.io](https://release-0-3.cluster-api.sigs.k8s.io)
 
+
+
 </aside>
 
 ## Why build Cluster API?