Update cloudflare.md

missing clear instructions on where to find the chart, without this the upgrade command will not work.

docs/tutorials/cloudflare.md

@@ -67,6 +67,14 @@ env:
 
 Finally, install the ExternalDNS chart with Helm using the configuration specified in your values.yaml file:
 
+```shell
+helm repo add external-dns https://kubernetes-sigs.github.io/external-dns/
+```
+
+```shell
+helm repo update
+```
+
 ```shell
 helm upgrade --install external-dns external-dns/external-dns --values values.yaml
 ```