How to make my player object increase its speed as it moves.

I am making a game based on speed. It is a ball that moves slowly at the start of a level, but increases in speed as it moves. Only problem is, I have no idea how to make in increase in speed with the code I have written (Based on the Brackeys make a game tutorial). Here is the code I have. I would also like the incline of platforms to affect the speed. Thank you in advance!

 #pragma strict
 
 var rotationSpeed = 100;
 var jumpHeight = 4;
 
 private var isFalling = false;
 
 function Update () 
 {
     //Handle ball rotation
     var rotation : float = Input.GetAxis ("Horizontal") * rotationSpeed;
     rotation *= Time.deltaTime;
     rigidbody.AddRelativeTorque (Vector3.back * rotation);
     
     //Handle Jump
     if (Input.GetKeyDown(KeyCode.Space) && isFalling == false)
     {
         rigidbody.velocity.y = jumpHeight;
         isFalling = true;
     }    
 }
 
 function OnCollisionStay ()
 {
     isFalling = false;
 }