update-docs(create-record-set)

added docs which explains how to create record set in Route53 using
annotations in ingress object

Author: Luvpreet Singh

docs/guide/external-dns/setup.md

@@ -41,4 +41,14 @@ Adequate roles and policies must be configured in AWS and available to the node(
     ```
     time="2017-09-19T02:51:54Z" level=info msg="config: &{Master: KubeConfig: Sources:[service ingress] Namespace: FQDNTemplate: Compatibility: Provider:aws GoogleProject: DomainFilter:[] AzureConfigFile:/etc/kuberne tes/azure.json AzureResourceGroup: Policy:upsert-only Registry:txt TXTOwnerID:my-identifier TXTPrefix: Interval:1m0s Once:false DryRun:false LogFormat:text MetricsAddress::7979 Debug:false}"
     time="2017-09-19T02:51:54Z" level=info msg="Connected to cluster at https://10.3.0.1:443"
-    ```
+    ```
+
+## Usage:
+To create a record set in the subdomain, from your ingress which has been created by the ingress-controller, simply add the following annotation in the ingress object specification,
+
+  annotations:
+    kubernetes.io/ingress.class: alb
+    alb.ingress.kubernetes.io/scheme: internet-facing
+
+    # for creating record-set
+    external-dns.alpha.kubernetes.io/hostname: my-app.test-dns.com