Add SPM installation to readme (airsidemobile#224)

* Update readme

* Format package.swift

* Fix typo

Author: Daniel

Package.swift

@@ -5,16 +5,10 @@ let package = Package(
     name: "JOSESwift",
     platforms: [.iOS(.v10)],
     products: [
-        .library(
-            name: "JOSESwift",
-            targets: ["JOSESwift"])
-    ],
-    dependencies: [
+        .library(name: "JOSESwift", targets: ["JOSESwift"])
     ],
+    dependencies: [],
     targets: [
-        .target(
-            name: "JOSESwift",
-            path: "JOSESwift"
-        )
+        .target(name: "JOSESwift", path: "JOSESwift")
     ],
     swiftLanguageVersions: [.v5])

README.md

@@ -109,6 +109,10 @@ github "airsidemobile/JOSESwift"
 
 Then build it by running `carthage update` and drag the built framework into your Xcode project. More documentation on using Carthage can be found [here](https://github.com/Carthage/Carthage).
 
+### Swift Package Manager
+
+To integrate JOSESwift in your Xcode project as a Swift package, follow Apple's article on how to [add package dependencies to your app](https://developer.apple.com/documentation/xcode/adding_package_dependencies_to_your_app).
+
 ## Usage
 
 JOSESwift covers three functional aspects: