Mark package as macos only

Author: jakeheis

Package.swift

@@ -5,6 +5,9 @@ import PackageDescription
 
 let package = Package(
     name: "Shout",
+    platforms: [
+        .macOS(.v10_10)
+    ],
     products: [
         .library(name: "Shout", targets: ["Shout"]),
     ],