Merge pull request #342 from rxchell/add-unknown-command-message

Add message for unknown command [Enhancement]

docs/DeveloperGuide.md

@@ -1092,7 +1092,12 @@ testers are expected to do more *exploratory* testing.
 ## **Appendix: Planned enhancements**
 Team size: 5
 
-1.
+1. **When an invalid command is entered, suggest possible valid commands.**
+* **Flaw:** When a user types an unknown command, the result display box shows `Unknown Command`, which is 
+   not informative.
+* **Enhancement:** The result display box will suggest possible valid commands for the user to enter.
+* **Sample Input:** User enters: `sort`
+* **Expected Output:** The result display box shows `Unknown Command. Did you mean: list, find, clear, add, edit, delete, exit, or help?`
 
 2. **Modify date parsing to check for leap years**
 * **Flaw:** When a user enters February 29 for a non-leap year, the date automatically adjusts to February