add sys ref

Author: yennanliu

system_design/README.md

@@ -2,4 +2,7 @@
 
 ## Meterials
 - Books
-	- https://github.com/yennanliu/knowledge_base_repo/tree/master/book/system_design
+	- https://github.com/yennanliu/knowledge_base_repo/tree/master/book/system_design
+- Others
+	- Sys Degisn By Example : https://third-bit.com/sdxpy/build/	
+		- code: https://github.com/yennanliu/sdxpy