How to add decals to terrain texture?

If it's the standard Unity terrain, it's better to create a plane with the desired decal and place it slightly in front of the point it should appear. A common case is when you shot with raycast and want to place a bullet hole where the target was hit (weapon script):

 var holeDecal: GameObject;
 
 function Update(){
   if (Input.GetButtonDown("Fire1")){
     var hit: RaycastHit;
     if (Physics.Raycast(transform.position, transform.forward, hit)){
       // calculate rotation to align decal's up direction to the surface normal:
       var rot = Quaternion.FromToRotation(Vector3.up, hit.normal);
       // calculate position a little ahead the hit point: 
       var pos = hit.point + 0.001 * hit.normal;
       // create the hole decal:
       Instantiate(holeDecal, pos, rot);
     }
   }
 }