Update readme

README.md

@@ -27,6 +27,7 @@ Kingfisher is a lightweight, pure-Swift library for downloading and caching imag
 - [x] Prefetching images and showing them from cache later when necessary.
 - [x] Extensions for `UIImageView`, `NSImage` and `UIButton` to directly set an image from a URL.
 - [x] Built-in transition animation when setting images.
+- [x] Customizable placeholder while loading images.
 - [x] Extensible image processing and image format support.
 
 The simplest use-case is setting an image to an image view with the `UIImageView` extension:
@@ -41,10 +42,11 @@ Kingfisher will download the image from `url`, send it to both the memory cache
 ## Requirements
 
 - iOS 8.0+ / macOS 10.10+ / tvOS 9.0+ / watchOS 2.0+
-- Swift 3 (Kingfisher 3.x), Swift 2.3 (Kingfisher 2.x)
+- Swift 4 (Kingfisher 4.x), Swift 3 (Kingfisher 3.x)
 
-Main development of Kingfisher will support Swift 3. Only critical bug fixes will be made for Kingfisher 2.x.
+Main development of Kingfisher is based on Swift 4. Only critical bug fixes will be applied to Kingfisher 3.x.
 
+Kingfisher 4.0 Migration - Kingfisher 3.x should be source compatible to Kingfisher 4. The reason for a major update is that we need to specify the Swift version explicitly for Xcode. All deprecated methods in Kingfisher 3 has been removed, so please ensure you have no warning left before you migrate from Kingfisher 3 to Kingfisher 4. If you have any trouble in migrating, please open an issue to discuss.
 [Kingfisher 3.0 Migration Guide](https://github.com/onevcat/Kingfisher/wiki/Kingfisher-3.0-Migration-Guide) - If you are upgrading to Kingfisher 3.x from an earlier version, please read this for more information.
 
 ## Next Steps