docs: update README grafana example config (#781)

Think the prometheus_grafana services are off by default, since I had to
modify the config this way to get grafana and prometheus to start.

Signed-off-by: Samuel Laferriere <samlaf92@gmail.com>

README.md

@@ -961,6 +961,9 @@ participants:
     cl_type: lighthouse
     count: 2
 snooper_enabled: true
+additional_services:
+  - prometheus_grafana
+ethereum_metrics_exporter_enabled: true
 ```
 
 </details>