Added material to Promises subsection

Author: eleev

README.md

@@ -200,6 +200,8 @@ getting-started)
 ### Promise
 - [Promises in Swift: Asynchronous Promises: Proof of Concept in Swift](https://medium.com/@robringham/promises-in-swift-66f377c3e403)
 - [Promise Pattern on Swift with PromiseKit And why you should migrate your async calls to them](https://hackernoon.com/promise-pattern-on-swift-with-promisekit-620efad7a5bd)
+- [Under The Hood Of Futures And Promises In Swift](https://www.swiftbysundell.com/posts/under-the-hood-of-futures-and-promises-in-swift)
+
 
 ### Redux
 - [Introduction to ReSwift that is a small framework that helps you to create Redux-like architectures in Swift](https://www.raywenderlich.com/155815/reswift-tutorial-memory-game-app)