Add platforms to Package.swift

Author: ikesyo

Package.swift

@@ -3,6 +3,9 @@ import PackageDescription
 
 let package = Package(
     name: "APIKit",
+    platforms: [
+        .macOS(.v10_10), .iOS(.v8), .tvOS(.v9), .watchOS(.v2)
+    ],
     products: [
         .library(name: "APIKit", targets: ["APIKit"]),
     ],