How to double jump

Hi, ive been scouring the internet left and right to put double jumps into my code,i'm making a 3D platformer, using a 3rd person controller and c#. I would like to know what additional script for the double jumping ill have to use and where abouts to include it.

Thanks. I'm quite new to this.

using UnityEngine; using System.Collections;

public class PlayerController : MonoBehaviour {

 public CharacterController controller;
 public GameObject projectile;

 Vector3 movementVector = Vector3.zero;
 //movement variables
 public float moveForce = 0;
 public float moveDamping = 0.5f;
 public float moveForceToAdd = 1f;
 public float movementSpeed = 2f;

 //jumping variables
 public float jumpForce = 1.0f;
 public float jumpDamping = 0.5f;
 public float jumpForceToAdd = 100f;
 int jumpCount = 1;

 bool grounded = false;

 // Use this for initialization
 void Start () {
 int result =     AddNumber (2, 3);
     Debug.Log (result);
 }
 
 // Update is called once per frame
 void Update () 
 {
     if (Input.GetKeyDown (KeyCode.Q)) 
     {
         Instantiate (projectile, transform.position + transform.forward * 2, transform.rotation);
     }
     movementVector = new Vector3 (Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));

     if (Input.GetAxis ("Vertical") != 0 || Input.GetAxis ("Horizontal") != 0) 
     {
         moveForce += moveForceToAdd;
     }

     if (Input.GetKeyDown (KeyCode.Space)) 
     {
         if (controller.isGrounded)
         {
             jumpForce += jumpForceToAdd;
         }
     }
     movementVector *= moveForce;
     movementVector += Physics.gravity;
     movementVector += new Vector3 (0, jumpForce, 0);
     gameObject.GetComponent<CharacterController>().Move (movementVector * Time.deltaTime);

     jumpForce *= jumpDamping;
     moveForce *= moveDamping;
 }
 int AddNumber (int a, int b)
 {
     return a + b;
     }

} `