Augmenting a speed linked to a Vector 2

Hey Guys,
I am making a game, and need some help.
I have this player that will under some circunstances go down. When it goes down I want it to act like flappy birds, by having its speed augmenting every frame.

	//Set the gravity
    public Vector2 gravity = new Vector2 (0, -10);

    //Makes the player fall while the space bar is not pressed
    //How can I make it every frame go down faster?
	if (Jumping == false) 
	{
		rigidbody2D.velocity = Vector2.zero;
		rigidbody2D.AddForce(gravity);
		if (Input.GetKeyDown(KeyCode.Space)){
			Jumping = true;
		}
	}

I am new to C# and Unity, so I really need some help and maybe an explication.

And how can I know my game FPS? How can I change it?
Thanks.

public float VerticalAcceleration = -2.0f;
public float VerticalSpeedMax = 10.0f;

private Vector2 _currentSpeed = Vector2.zero;

and then, instead of your rigidbody code:

_currentSpeed.y += VerticalAcceleration * Time.deltaTime;
_currentSpeed.y = Mathf.Clamp(_currentSpeed.y, -VerticalSpeedMax, VerticalSpeedMax);
rigidbody2D.velocity = _currentSpeed * Time.deltaTime;

Cheers!