Add recommendation for using readline library in course-definition.yml

Author: andy1li

course-definition.yml

@@ -1444,6 +1444,10 @@ stages:
 
       The tester will expect the previous commands to be displayed when the up arrow key is pressed.
 
+      ### Notes
+
+      - We recommend using a library like [readline](https://en.wikipedia.org/wiki/GNU_Readline) for your implementation. Most modern shells and REPLs (like the Python REPL) use readline under the hood. While you may need to override some of its default behaviors, it's typically less work than starting from scratch.
+
     marketing_md: |-
       In this stage, you'll implement support for recalling history with the up arrow key.