add overview diagram

Signed-off-by: Arvind Thirumurugan <arvindth@microsoft.com>

Author: Arvind Thirumurugan

approval-controller-metric-collector/README.md

@@ -8,6 +8,8 @@ This directory contains two controllers:
 - **approval-request-controller**: Runs on the hub cluster to automate approval decisions for staged updates
 - **standalone-metric-collector**: Runs on member clusters to collect workload health metrics from Prometheus
 
+![Approval Controller and Metric Collector Architecture](approval-controller-metric-collector.drawio.png)
+
 ## Prerequisites
 
 - Docker or Podman for building images