update readme

jincheng9 · jincheng9 · commit bf4ed0b09729 · 2021-10-28T15:12:23.000+08:00
diff --git a/readme.md b/readme.md
@@ -14,4 +14,5 @@
 * [lesson12: 结构体](./workspace/lesson12)
 * [lesson13: 切片Slice](./workspace/lesson13)
 * [lesson14: range迭代](./workspace/lesson14)
+* [lesson15: map集合](./workspace/lesson15)
 
