Update README.md

Author: Joseph Luce

system_design/README.md

@@ -2,6 +2,7 @@
 - http://highscalability.com/
 
 ### How would you design a messenger app?
+I recommend checking this guy's channel
 - https://www.youtube.com/watch?v=5m0L0k8ZtEs
 
 ### Rest Api and HTTP