Update azure.md

For this to run, VNET and SUBNET need to be set.

docs/getting-started-guides/azure.md

@@ -20,6 +20,13 @@ Next build Kubernetes, package the release, and upload to Azure Storage:
     cd kubernetes
     release/azure/release.sh
 
+Next, specify an existing US West virtual network in /cluster/azure/config.sh:
+
+    AZ_VNET=<vnet name>
+    AZ_SUBNET=<subnet name>
+    
+Now you're ready.
+
 You can then use the `cluster/kube-*.sh` scripts to manage your azure cluster, start with:
 
     cluster/kube-up.sh