Added platforms in PackageDescriptor

Author: zaniluca

Package.swift

@@ -5,6 +5,12 @@ import PackageDescription
 
 let package = Package(
     name: "SwiftUISnackbar",
+    platforms: [
+            .macOS(.v11),
+            .iOS(.v14),
+            .tvOS(.v14),
+            .watchOS(.v7)
+        ],
     products: [
         // Products define the executables and libraries a package produces, and make them visible to other packages.
         .library(