How do I add more text parts to this script?

Hi guys, So I'm making the introduction to my game, and I wanted it to be letter by letter, so I'm using this script:

 using UnityEngine;
 using System.Collections;
 public class mainText : MonoBehaviour
 {
     public float letterPause = 0.05f;
     public AudioClip sound;
     public GUIStyle font;
     string message;
     string text;
   
     void Start ()
     {
         message = "During the 18th century, waves of vicious thieves washed over the southern" +
             " part of the land now known as the Netherlands, and the adjacent Belgium and German" +
             " Rhineland. \n \nGroups of men and women of all kinds set out to rob rich farmers," +
             " civillians, churches, and monasteries. \n \nThese godless robbers were commonly" +
             " associated with the devil himself.";
         text = "";
         StartCoroutine(TypeText());
     }
   
     IEnumerator TypeText ()
     {
         foreach (char letter in message.ToCharArray())
         {
             text += letter;
             if (sound)
                 audio.PlayOneShot (sound);
             yield return 0;
             yield return new WaitForSeconds (letterPause);
         }
     }
   
     void OnGUI()
     {
         GUI.Box(new Rect(Screen.width / 2 - 320, Screen.height / 2 - 240, 640, 480), text, font);
     }
   
     void Update()
     {
         if(Input.GetKeyDown (KeyCode.Return))
         {
             StopAllCoroutines();
             text = message;
         }
     }
 }

But does anyone know how I can create a button that when clicked on, shows the next part of the text?