Added SPM support

Package.swift

@@ -0,0 +1,28 @@
+// swift-tools-version:5.0
+import PackageDescription
+
+let package = Package(
+    name: "CupertinoJWT",
+    platforms: [
+        .macOS(.v10_12),
+        .iOS(.v10),
+        .tvOS(.v10),
+        .watchOS(.v3)
+    ],
+    products: [
+        .library(
+            name: "CupertinoJWT",
+            targets: ["CupertinoJWT"]),
+    ],
+    dependencies: [],
+    targets: [
+        .target(
+            name: "CupertinoJWT",
+            dependencies: [],
+            path: "Sources"),
+        .testTarget(
+            name: "CupertinoJWTTests",
+            dependencies: ["CupertinoJWT"]),
+    ],
+    swiftLanguageVersions: [.v5]
+)

README.md

@@ -58,6 +58,10 @@ github "ethanhuang13/CupertinoJWT"
 [Carthage Installation]: https://github.com/Carthage/Carthage#installing-carthage
 [Carthage Usage]: https://github.com/Carthage/Carthage#adding-frameworks-to-an-application
 
+### SPM
+
+When using Xcode 11 you can install `CupertinoJWT` by going to your Project settings > `Swift Packages` and add the repository by providing the GitHub URL. Or go to `File` > `Swift Packages` > `Add Package Dependencies...`
+
 ## What's this all about?
 
 Apple has several server APIs uses JSON Web Token([JWT](https://jwt.io)) as authentication method, including [Apple Push Notification service (APNs)](https://developer.apple.com/library/archive/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/CommunicatingwithAPNs.html#//apple_ref/doc/uid/TP40008194-CH11-SW1), [MusicKit](https://help.apple.com/developer-account/#/devce5522674), [DeviceCheck](https://help.apple.com/developer-account/#/devc3cc013b7) and [App Store Connect API](https://developer.apple.com/videos/play/wwdc2018/303/). Probably more in the future.

Tests/CupertinoJWTTests.swift → Tests/CupertinoJWTTests/CupertinoJWTTests.swift

@@ -41,4 +41,10 @@ hMdWYrKn
             XCTFail(error.localizedDescription)
         }
     }
+
+	static var allTests : [(String, (MemoryCacheTests) -> () throws -> Void)] {
+		return [
+			("testJWT", testJWT),
+		]
+	}
 }

Tests/LinuxMain.swift

@@ -0,0 +1,6 @@
+import XCTest
+@testable import CupertinoJWTTests
+
+XCTMain([
+    testCase(CupertinoJWTTests.allTests),
+])