add help

src/include/builtins.h

@@ -32,6 +32,12 @@ bool is_builtin(char *command);
  */
 void run_builtin(StringArray *args);
 
+/**
+ * Show help message for the shell.
+ * @param args The arguments passed to help.
+ */
+void help(StringArray *args);
+
 /**
  * Exit the shell.
  * @param args The arguments that were used to call exit. This is used to free the memory before exit.