Document when each part of the API was added
For those that are trying to target old versions of sketch it's incredibly useful to know when I have to drop down to some sketch headers / appkit methods. 

For example, in Apple's documentation there is a little SDK version number to help you know what version of MacOS it is supported with.
<img width="967" alt="Screen Shot 2019-03-11 at 6 25 22 PM" src="https://user-images.githubusercontent.com/4199296/54168353-45fcf900-442b-11e9-9011-3df085a1215d.png">
