add Swift Package Manager support

Authored-by: Arkayn Gamer <[email protected]>

Package.swift

@@ -0,0 +1,34 @@
+// swift-tools-version:5.3
+
+//
+//  Package.swift
+//  IAPReceiptVerifier
+//
+//  Created by Mark Hoath on 3/7/21.
+//
+
+import PackageDescription
+
+let package = Package (
+    name: "IAPReceiptVerifier",
+    platforms: [
+            .iOS(.v14)
+        ],
+    products: [
+            // Products define the executables and libraries a package produces, and make them visible to other packages.
+            .library(
+                name: "IAPReceiptVerifier",
+                targets: ["IAPReceiptVerifier"])
+        ],
+    dependencies: [
+            // Dependencies declare other packages that this package depends on.
+    ],
+    targets: [
+            // Targets are the basic building blocks of a package. A target can define a module or a test suite.
+            // Targets can depend on other targets in this package, and on products in packages this package depends on.
+            .target(
+                name: "IAPReceiptVerifier",
+                dependencies: [],
+                path: "IAPReceiptVerifier/Classes")
+    ]
+)