Add future reading section to client settings doc

Author: Kate Osborn

site/content/how-to/traffic-management/client-settings.md

@@ -276,3 +276,7 @@ spec:
     maxSize: "75" # sizes without a unit are bytes.
 EOF
 ```
+
+## Further Reading
+
+- [Custom policies]({{< relref "overview/custom-policies.md" >}}): learn about how NGINX Gateway Fabric custom policies work.