add help command

src/main/java/duke/command/HelpCommand.java

@@ -0,0 +1,13 @@
+package duke.command;
+
+import duke.exception.DukeException;
+import duke.stack.CallStack;
+import duke.task.TaskList;
+import duke.util.Constants;
+
+public class HelpCommand extends Command {
+    @Override
+    public String executeCommand(TaskList taskList, CallStack callStack) throws DukeException {
+        return Constants.HELP_MESSAGE;
+    }
+}

src/main/java/duke/util/Constants.java

@@ -10,7 +10,8 @@
 public class Constants {
     public static final String HORIZONTAL_LINE = "____________________________________________________________";
 
-    public static final String GREETINGS = "Hello there, I am Giga Chad Duke\nHow can I help you?";
+    public static final String GREETINGS = "Hello there, I am Giga Chad Duke.\nHow can I help you?\nType help to list " +
+            "all the commands.";
 
     public static final String BYE = "Bye, hope to see you again soon!";
 
@@ -25,4 +26,15 @@ public class Constants {
 
     public static final int DELAY = 400;
 
+    public static final String HELP_MESSAGE = "Here are the list of commands:\n" +
+            "1. list: list out all of your tasks\n" +
+            "2. todo <task>: add a todo task into your task list\n" +
+            "3. deadline <task> /by <deadline>: add a deadline into your task list\n" +
+            "4. even <task> /at <time>: add an event into your task list\n" +
+            "5. mark <inde>: mark a task at specified index as done\n" +
+            "6. unmark <index>: unmark a task at specified index as not done yet\n" +
+            "7. delete <index>: delete a task at sepcified index\n" +
+            "8. find <keyword>: find a task that containst keyword\n" +
+            "9. undo: undo your last action\n" +
+            "10. bye: termintate this program";
 }

src/main/java/duke/util/Parser.java

@@ -47,6 +47,8 @@ public static Command parse(String line) throws DukeException {
             return new DeleteCommand(taskId);
         case "undo":
             return new UndoCommand();
+            case "help":
+                return new HelpCommand();
         default:
             throw new CommandNotFoundException();
         }