Add the content for how services work

Signed-off-by: yuexiao-wang <wang.yuexiao@zte.com.cn>

docs/swarm/how-swarm-mode-works/menu.md

@@ -15,3 +15,4 @@ weight=11
 ## TOC
 
 * [How nodes work](nodes.md)
+* [How services work](services.md)