Add description for SPM API version 4 (#2341)

* Add description for SPM API version 4

* Fix marking of API difference

README.md

@@ -129,12 +129,22 @@ The [Swift Package Manager](https://swift.org/package-manager/) is a tool for au
 
 Once you have your Swift package set up, adding Alamofire as a dependency is as easy as adding it to the `dependencies` value of your `Package.swift`.
 
+#### Swift 3
+
 ```swift
 dependencies: [
     .Package(url: "https://github.com/Alamofire/Alamofire.git", majorVersion: 4)
 ]
 ```
 
+#### Swift 4
+
+```swift
+dependencies: [
+    .package(url: "https://github.com/Alamofire/Alamofire.git", from: "4.0.0")
+]
+```
+
 ### Manually
 
 If you prefer not to use any of the aforementioned dependency managers, you can integrate Alamofire into your project manually.