docs: point to cluster-platform repo for platform services

cluster-bootstrap only brings up the minimal layer (argocd, metallb,
envoy-gateway, storage). App/platform services now live in a separate
cluster-platform repo, applied after this one finishes.

README.md

@@ -321,3 +321,16 @@ cluster-bootstrap/
     └── nfs-provisioner/
         └── values.yaml              # ⚠ fill in nfs.server + nfs.path before push
 ```
+
+---
+
+## Next: platform services
+
+This repo only brings up the minimal layer needed to make the cluster usable
+(ArgoCD, LoadBalancer IPs, HTTP gateway, storage). Application/platform
+services (Vault, external-secrets, Harbor, Jenkins, SonarQube, etc.) live in
+the separate `cluster-platform` repo, applied after this one finishes:
+
+```bash
+kubectl apply -f ../cluster-platform/platform-app.yaml
+```