Add Swift Package Manager to README (#234)

We have added support for Swift Package Manager so this PR adds a section to the docs explaining how to import PusherSwift using Swift Package Manager.

Author: TomKemp

README.md

@@ -108,6 +108,38 @@ To integrate PusherSwift into your Xcode project using Carthage, specify it in y
 github "pusher/pusher-websocket-swift"
 ```
 
+### Swift Package Manager
+
+To integrate PusherSwift into your project using [Swift Package Manager](https://swift.org/package-manager/), you can add the library as a dependency in Xcode (11 and above) – see the [docs](https://developer.apple.com/documentation/xcode/adding_package_dependencies_to_your_app). The package repository URL is:
+
+```bash
+https://github.com/pusher/pusher-websocket-swift.git
+```
+
+Alternatively, you can add PusherSwift as a dependency in your `Package.swift` file. For example:
+
+```swift
+// swift-tools-version:5.1
+import PackageDescription
+
+let package = Package(
+    name: "YourPackage",
+    products: [
+        .library(
+            name: "YourPackage",
+            targets: ["YourPackage"]),
+    ],
+    dependencies: [
+        .package(url: "https://github.com/pusher/pusher-websocket-swift.git", from: "7.2.0"),
+    ],
+    targets: [
+        .target(
+            name: "YourPackage",
+            dependencies: ["PusherSwift"]),
+    ]
+)
+```
+
 ## Configuration
 
 There are a number of configuration parameters which can be set for the Pusher client. For Swift usage they are: