Added Computed Properties and Property Observers to Swift Cheatsheet. 

Computed Properties and Property observers are two things that are always tripping me up in Swift. I always think "I want a custom setter and getter and reach for the `set` `get` when really I should be using `willSet` or `didSet`. 

I was so excited to see that the Dash Cheatsheets were up on github and that I could contribute. 

This is basically my first pull request, If I have done this incorrectly please let me know I willing to put in the work to be part of this community. 

Thanks for Dash! really enjoying it.