Update README.md

Author: hzlgit

README.md

@@ -197,7 +197,16 @@ UIUserNotificationSettings *settings = [UIUserNotificationSettings settingsForTy
 ```
 
 #### Android配置
+在`android/app/build.gradle`里，defaultConfig栏目下添加如下代码：
+```
+manifestPlaceholders = [
+    ......
 
+    "NIM_KEY": "云信appId"
+ 
+    ......
+]
+```
 在`AndroidManifest.xml`里，添加如下代码：
 ```
 < manifest