Finished Swift 4 introduction.md

Author: Bob Lee

course/swift4/introduction.md

@@ -1,2 +1,7 @@
 # Chapter 9
 ## What's New in Swift 4
+Welcome to the second last chapter of Learn Swift with Bob. It's amazing to see you all the way here. Relatively, the update from Swift 3 to Swift 4 isn't significant as I personally have faced from Swift 2 to Swift 3. Swift 3 code still runs in Swift 4 so I didn't technically have to re-record the entire video, but I wanted to make sure I stay on the top of the game.
+
+In the first lesson, you will learn how apple engineers protect their source code through access control and you can achieve the same goal when distributing your work especially in open source projects. In lesson 2, you will learn how to apply generics to subscripts which we covered in the very first chapter in this course. Then, you will learn how to convert a json to object vice versa in a single line of code using `codable`. If you are not familiar with the json format, that's okay. I will get you started. Lastly, you will learn how to combine functional programming with Swift dictionary.
+
+By the end of this chapter, you will never code the same way as you did. Well, hopefully that has been the case for each chapter. Let's get started right now.