assets: add wechat group

see #75

assets/community.md

@@ -17,3 +17,9 @@ The book offers a telegram chat group, feel free to join if you are interested:
 ![](../assets/qq-group.png)
 
 > 提示: `bW9kZXJuLWNwcC10dXRvcmlhbC1naXRodWI=`
+
+本书还提供以下读者微信交流群，有兴趣的读者可以加入，加群请扫描如下二维码：
+
+![](../assets/wechat-group.jpg)
+
+若群已加满，请联系并添加 `oulongqi` 并注明『现代 C++ 教程』获取入群邀请。

website/Makefile

@@ -5,6 +5,7 @@ all: clean
 	cp ../assets/cover-2nd-en.png ./src/modern-cpp/assets/cover-2nd-en.png
 	cp ../assets/alipay.jpg ./src/modern-cpp/assets/alipay.jpg
 	cp ../assets/wechat.jpg ./src/modern-cpp/assets/wechat.jpg
+	cp ../assets/wechat-group.jpg ./src/modern-cpp/assets/wechat-group.jpg
 	cp ../assets/qq-group.png ./src/modern-cpp/assets/qq-group.png
 	cp ../assets/donate.md ./src/modern-cpp/about/
 	cp ../assets/community.md ./src/modern-cpp/about/
@@ -20,6 +21,7 @@ clean:
 		./src/modern-cpp/assets/figures \
 		./src/modern-cpp/assets/alipay.jpg \
 		./src/modern-cpp/assets/wechat.jpg \
+		./src/modern-cpp/assets/wechat-group.jpg \
 		./src/modern-cpp/about/donate.md \
 		./src/modern-cpp/about/community.md \
 		./src/modern-cpp/code \