docs: add service annotation patching to example kustomization (#3020)

kustomize/README.md

@@ -13,10 +13,24 @@ configMapGenerator:
   - name: mc
     envs:
       - mc.env
+patches:
+  # Example of using a patch to set external service name for mc-router to pick up
+  - path: set-external-servername.yml
 ```
 
 ### mc.env
 ```
 EULA=true
 TYPE=FORGE
+```
+
+###
+
+```yaml
+apiVersion: v1
+kind: Service
+metadata:
+  name: mc
+  annotations:
+    mc-router.itzg.me/externalServerName: forge.example.com
 ```