Update 2014-05-08-animations-explained.md

"Setting the additive property to YES tells Core Animation to add the values of the animation to the value of the model layer, before updating the presentation layer"

```
CAKeyframeAnimation *animation = [CAKeyframeAnimation animation];
animation.keyPath = @"position.x";
animation.values = @[ @0, @10, @-10, @10, @0 ];
animation.keyTimes = @[ @0, @(1 / 6.0), @(3 / 6.0), @(5 / 6.0), @1 ];
animation.duration = 0.4;

animation.additive = YES;

[form.layer addAnimation:animation forKey:@"shake"];
```

above code doesn't change the value of model layer. I think it would be better if we write something like 

"Setting the additive property to YES tells Core Animation to add the values of the animation to the value of the current presentation layer"