added flutter course (#10956)

Co-authored-by: Min Han Kyaw <mhk@mit.com.mm>

courses/free-courses-zh.md

@@ -1,6 +1,7 @@
 ### Index
 
 * [0 - MOOC](#0---mooc)
+* [Flutter](#flutter)
 * [Linux](#linux)
 
 
@@ -9,6 +10,11 @@
 * [freeCodeCamp](https://chinese.freecodecamp.org)
 
 
+### Flutter
+
+* [Flutter 仿微信朋友圈](https://www.youtube.com/playlist?v=7lZRWWELIaA&list=PL274L1n86T80VQcJb76zcXcPpF-S-fFV-) - ducafecat
+
+
 ### Linux
 
 * [Linux 核心設計](https://youtube.com/playlist?list=PL6S9AqLQkFpongEA75M15_BlQBC9rTdd8) - jserv