add SEN_AUTH.md

docs/SEND_AUTH.md

+## 发送Auth验证
+```Fluwx```目前只支持获取```code```，若要获取```access_token```请在服务器端完成。
+```dart
+   fluwx.sendAuth(WeChatSendAuthModel(
+      scope: "snsapi_userinfo",
+          state:"wechat_sdk_demo_test",
+    ));
+```

example/lib/share_image_page.dart

@@ -17,10 +17,6 @@ class _ShareImagePageState extends State<ShareImagePage> {
   void initState() {
     super.initState();
     fluwx = new Fluwx();
-    fluwx.sendAuth(WeChatSendAuthModel(
-      scope: "snsapi_userinfo",
-          state:"wechat_sdk_demo_test",
-    ));
   }
   @override
   Widget build(BuildContext context) {

lib/src/models/wechat_send_auth_model.dart

+import 'package:flutter/foundation.dart';
 class WeChatSendAuthModel {
   final String scope;
   final String state;
   final String openId;
 
-  WeChatSendAuthModel(this.scope, this.state,this.openId) :
+  WeChatSendAuthModel({
+    @required this.scope,
+    this.state,
+    this.openId }) :
         assert(scope != null && scope
             .trim()
             .isNotEmpty);