Add basic SwiftPM manifest file

Package.swift

@@ -0,0 +1,16 @@
+// swift-tools-version:4.2
+import PackageDescription
+
+let package = Package(
+    name: "SwiftyStoreKit",
+    // platforms: [.iOS("8.0"), .macOS("10.10"), tvOS("9.0"), .watchOS("2.0")],
+    products: [
+        .library(name: "SwiftyStoreKit", targets: ["SwiftyStoreKit"])
+    ],
+    targets: [
+        .target(
+            name: "SwiftyStoreKit",
+            path: "SwiftyStoreKit"
+        )
+    ]
+)