update the readme about SwiftPM

Author: dreampiggy

Package.resolved

@@ -5,6 +5,7 @@
 [![License](https://img.shields.io/cocoapods/l/SDWebImagePhotosPlugin.svg?style=flat)](https://cocoapods.org/pods/SDWebImagePhotosPlugin)
 [![Platform](https://img.shields.io/cocoapods/p/SDWebImagePhotosPlugin.svg?style=flat)](https://cocoapods.org/pods/SDWebImagePhotosPlugin)
 [![Carthage compatible](https://img.shields.io/badge/Carthage-compatible-4BC51D.svg?style=flat)](https://github.com/SDWebImage/SDWebImagePhotosPlugin)
+[![SwiftPM compatible](https://img.shields.io/badge/SwiftPM-Compatible-brightgreen.svg)](https://swift.org/package-manager/)
 [![codecov](https://codecov.io/gh/SDWebImage/SDWebImagePhotosPlugin/branch/master/graph/badge.svg)](https://codecov.io/gh/SDWebImage/SDWebImagePhotosPlugin)
 
 ## What's for
@@ -39,6 +40,18 @@ SDWebImagePhotosPlugin is available through [Carthage](https://github.com/Cartha
 github "SDWebImage/SDWebImagePhotosPlugin"
 ```
 
+#### Swift Package Manager
+
+SDWebImagePhotosPlugin is available through [Swift Package Manager](https://swift.org/package-manager).
+
+```swift
+let package = Package(
+    dependencies: [
+        .package(url: "https://github.com/SDWebImage/SDWebImagePhotosPlugin.git", from: "0.3")
+    ]
+)
+```
+
 ## Usage
 **Important!** To use Photos Library plugin. Firstly you need to register the photos loader to image manager.