Added support for SwiftPM

Package.swift

@@ -0,0 +1,25 @@
+// swift-tools-version:5.3
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+
+import PackageDescription
+
+let package = Package(
+    name: "Sodium",
+    platforms: [
+        .macOS(.v10_10), .iOS(.v9)
+        ],
+    products: [
+        // Products define the executables and libraries produced by a package, and make them visible to other packages.
+        .library(
+            name: "Sodium",
+            targets: ["Sodium"]
+        )
+    ],
+    targets: [
+        .binaryTarget(
+            name: "Sodium",
+            url: "https://github.com/OuterCorner/Sodium/releases/download/1.0.18+201014.0/Sodium.xcframework.zip",
+            checksum: "ee4dadc153329ab5b3f11aa73a64ed677be1026017819f7b3b198fc4d5c35986"
+        )
+    ]
+)

README.md

@@ -17,17 +17,20 @@ You have a few different options:
  *  Include the Sodium.xcodeproj as a dependency in your project. This is what the projects under ```Examples/``` are doing. Doing this means Sodium.framework will be compiled alongside your project, including after every clean.  
  *  Use a pre-built Sodium.framework. You can find them under [Releases](https://github.com/OuterCorner/Sodium/releases).
 
-### Carthage
+### SwiftPM
 
-Add Sodium as a dependency on your ```Cartfile```:
-
-```
-github "OuterCorner/Sodium"
+In your `Package.swift`, add `Sodium` as a dependency:
+```swift
+dependencies: [
+  .package(url: "https://github.com/OuterCorner/Sodium", from: "1.0.17")
+],
 ```
-And run:
 
-```
-carthage update
+Associate the dependency with your target:
+```swift
+targets: [
+  .target(name: "App", dependencies: ["Sodium"])
+]
 ```
 
 ## Usage