Added support of Swift Package Manager

This allows to add fmt as dependency to other Swift Packages or Xcode projects

.gitignore

@@ -22,3 +22,4 @@ cmake_install.cmake
 fmt-*.cmake
 fmt.pc
 virtualenv
+.swiftpm/xcode

Package.swift

@@ -0,0 +1,31 @@
+
+// swift-tools-version: 5.8
+
+import PackageDescription
+
+let package = Package(
+    name: "fmt",
+    platforms: [
+        .macOS(.v11),
+        .iOS(.v13)
+    ],
+    products: [
+        .library(
+            name: "fmt",
+            targets: ["fmt"]),
+    ],
+    targets: [
+        .target(name: "fmt",
+                path: "",
+                exclude:[
+                    // module library is not needed for swift package
+                    "src/fmt.cc",
+                ],
+                sources:[
+                    "src"
+                ],
+                publicHeadersPath:"include"
+               )
+    ],
+    cxxLanguageStandard: .cxx20
+)