add auth counter_v2 interface doc

doc/apfree-wifidog-counter-v2接口说明.md

@@ -0,0 +1,17 @@
+1.1 上报数据接口定义
+authserv_path/auth/?stage=counters_v2
+如：
+wifidog/auth/?stage=counters_v2
+1.2 上报接口数据说明
+采用post的方式上传数据，一次性将所有移动端的信息上报，提高上报效率，降低服务器端的压力
+数据说明如下：
+```
+{"gw_id":"gw_id", clients:[
+{"id":id,"ip":"ipaddress","mac":"macaddress","token":"tokenvalue","channel_path":channelPath,
+"name":"clientname","incoming":incoming,"outgoing":outgoing,"first_login":first_login,
+"is_online":is_online,"wired":is_wired_device}, ....]}
+ ```
+ 2.1 返回数据说明
+ ```
+ {"gw_id":"gw_id","auth_op":[{"id":clt_id,"auth_code":authcode}, ....]}
+ ```