[Doc] update wechat qrcode

Author: chenjiahan

README.md

@@ -124,13 +124,13 @@ Modern browsers and Android 4.0+, iOS 6+.
 
 You can scan the following QR code to access the demo：
 
-<img src="https://img.yzcdn.cn/vant/preview_qrcode_20180528.png" width="250" height="250" >
+<img src="https://img.yzcdn.cn/vant/preview_qrcode_20180528.png" width="220" height="220" >
 
 ## Wechat Group
 
 Scan the qrcode to join our wechat discussion group, please note that you want to join Vant discussion group.
 
-<img src="https://img.yzcdn.cn/vant/wechat_20180606.png" width="250" >
+<img src="https://img.yzcdn.cn/vant/wechat_20180606.png" width="220" height="292" >
 
 ## LICENSE
 

README.zh-CN.md

@@ -133,9 +133,9 @@ Vue.use(Vant);
 
 ## 微信讨论群
 
-欢迎大家加入 Vant 交流群一起讨论，添加下方微信并注明『加入 Vant 交流群』即可
+欢迎大家加入 Vant 交流群一起讨论，添加下方微信并注明『加入 Vant 交流群』即可
 
-<img src="https://img.yzcdn.cn/vant/wechat_20180606.png" width="220" >
+<img src="https://img.yzcdn.cn/vant/wechat_20180606.png" width="220" height="292" >
 
 ## 开源协议