Smoothly increase rotation speed when pressing a key

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class Kruis : MonoBehaviour
 {
     public float RotationSpeed = 1;
     public float RotationAcceleration = 1;
     public float MaxRotationSpeed = 50;
     public KeyCode AccelerationKey = KeyCode.W;
     public KeyCode DecelerationKey = KeyCode.S;
     public Vector3 RotationAxis = Vector3.up;
     public bool RotateAroundWorldAxis;
     public bool RotateAntiClockWise;
     private float currentRotationSpeed;
 
     void Start()
     {
         currentRotationSpeed = RotationSpeed;
     }
 
     void Update()
     {
         if (Input.GetKey(AccelerationKey))
         {
             currentRotationSpeed += Time.deltaTime * RotationAcceleration;
         }
         if (Input.GetKey(DecelerationKey))
         {
             currentRotationSpeed -= Time.deltaTime * RotationAcceleration;
         }
 
         currentRotationSpeed = Mathf.Clamp(currentRotationSpeed, 0, MaxRotationSpeed);
 
         float angle = currentRotationSpeed;
         if (RotateAntiClockWise)
             angle = -angle;
 
         Vector3 rotationAxis = RotationAxis;
         if (!RotateAroundWorldAxis)
             rotationAxis = transform.rotation * RotationAxis;
         transform.RotateAround(transform.position, rotationAxis, angle);
     }
 
 #if UNITY_EDITOR
     private void OnDrawGizmosSelected()
     {
         Quaternion rotation       = RotateAroundWorldAxis ? Quaternion.identity : transform.rotation;
         Vector3 rotationAxis      = rotation * RotationAxis;
         float angle               = RotateAntiClockWise ? 1/2f : 1;
         float radius              = 0.5f;
         Color gizmosColor         = Color.magenta;
         Color handlesColor        = UnityEditor.Handles.color;
         Vector3 arrowCap          = transform.position + rotationAxis + transform.rotation * new Vector3(Mathf.Cos(angle * Mathf.PI), 0, Mathf.Sin(angle * Mathf.PI)) * radius;
         Vector3 leftArrowEnd      = arrowCap + Quaternion.Euler(0, RotateAntiClockWise ? 0 : 90, 0) * rotation * new Vector3(0.1f, 0, 0.1f) ;
         Vector3 rightArrowEnd     = arrowCap + Quaternion.Euler(0, RotateAntiClockWise ? 0 : 90, 0) * rotation * new Vector3(0.1f, 0, -0.1f);
         UnityEditor.Handles.color = gizmosColor;
         Debug.DrawLine(transform.position - rotationAxis, transform.position + rotationAxis, gizmosColor);
         UnityEditor.Handles.DrawWireArc(transform.position + rotationAxis, rotationAxis, transform.forward, 270, radius);
         Debug.DrawLine(arrowCap, leftArrowEnd, gizmosColor);
         Debug.DrawLine(arrowCap, rightArrowEnd, gizmosColor);
         UnityEditor.Handles.color = handlesColor;
     }
 #endif
 }

ORIGINAL ANSWER

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class Kruis : MonoBehaviour
 {
     public float RotationSpeed = 1;
     public float RotationAcceleration = 1;
     private float currentRotationSpeed;
 
     void Start()
     {
         currentRotationSpeed = RotationSpeed;
     }
 
     void Update()
     {
         if (Input.GetKeyDown("w") || Input.GetKeyDown("s"))
         {
             currentRotationSpeed = RotationSpeed;
         }
         if (Input.GetKey("w"))
         {
             transform.RotateAround(transform.position, transform.up, currentRotationSpeed);
             currentRotationSpeed += Time.deltaTime * RotationAcceleration;
         }
         if (Input.GetKey("s"))
         {
             transform.RotateAround(transform.position, transform.up, currentRotationSpeed);
             currentRotationSpeed += Time.deltaTime * RotationAcceleration;
         }
     }
 }