add section for core CAPI pre-release consumption

docs/book/src/clusterctl/commands/upgrade.md

@@ -123,3 +123,22 @@ clusterctl upgrade apply \
 In this case, all the provider's versions must be explicitly stated.
 
 </aside>
+
+<aside class="note warning">
+
+<h1> Upgrading to Cluster API core components pre-release versions </h1>
+
+Use `clusterctl` CLI options to target the [desired version](https://github.com/kubernetes-sigs/cluster-api/releases).  
+
+The following shows an example of upgrading `bootrap`, `kubeadm` and `core` components to version `v1.6.0-rc.1`:
+
+```bash
+TARGET_VERSION=v1.6.0-rc.1
+
+clusterctl upgrade apply \
+    --bootstrap=kubeadm:${TARGET_VERSION} \
+    --control-plane=kubeadm:${TARGET_VERSION} \
+    --core=cluster-api:${TARGET_VERSION}
+```
+
+</aside>