app 侧主叫呼叫流程图

Change-Id: Ie82234be112960e4c92ada8ea810ac9c0d4272c3

Author: sevenhhe

sdk/explorer-link-rtc/README.md

@@ -41,3 +41,9 @@ TRTCVideoCallActivity.startBeingCall(Context context, RoomKey roomKey, String be
 
 * 首页轮询TRTC设备状态查看App是否被呼叫
 可参考app module下的[requestDeviceList](https://github.com/tencentyun/iot-link-android/blob/master/app/src/main/java/com/tencent/iot/explorer/link/App.kt)接口
+
+## 通话流程梳理
+##### 连连 APP 呼叫 Android 设备端
+
+时序图：
+![UserCallDeviceUML](media/UserCallDeviceUML.png)