Bump Swift version to 5.2 (#85)

* Migrate to the Swift 5.2 package manifest format
* Also includes a dependency update, bumping Plot to version 0.8.0.
* README: Bump Swift version to 5.2 and add section about system requirements

Author: JohnSundell

Package.resolved

@@ -1,4 +1,4 @@
-// swift-tools-version:5.1
+// swift-tools-version:5.2
 
 /**
 *  Publish
@@ -15,12 +15,12 @@ let package = Package(
         .executable(name: "publish-cli", targets: ["PublishCLI"])
     ],
     dependencies: [
-        .package(url: "https://github.com/johnsundell/ink.git", from: "0.2.0"),
-        .package(url: "https://github.com/johnsundell/plot.git", from: "0.4.0"),
-        .package(url: "https://github.com/johnsundell/files.git", from: "4.0.0"),
-        .package(url: "https://github.com/johnsundell/codextended.git", from: "0.1.0"),
-        .package(url: "https://github.com/johnsundell/shellout.git", from: "2.3.0"),
-        .package(url: "https://github.com/johnsundell/sweep.git", from: "0.4.0")
+        .package(name: "Ink", url: "https://github.com/johnsundell/ink.git", from: "0.2.0"),
+        .package(name: "Plot", url: "https://github.com/johnsundell/plot.git", from: "0.4.0"),
+        .package(name: "Files", url: "https://github.com/johnsundell/files.git", from: "4.0.0"),
+        .package(name: "Codextended", url: "https://github.com/johnsundell/codextended.git", from: "0.1.0"),
+        .package(name: "ShellOut", url: "https://github.com/johnsundell/shellout.git", from: "2.3.0"),
+        .package(name: "Sweep", url: "https://github.com/johnsundell/sweep.git", from: "0.4.0")
     ],
     targets: [
         .target(

Package.swift

@@ -3,7 +3,7 @@
 </p>
 
 <p align="center">
-    <img src="https://img.shields.io/badge/Swift-5.1-orange.svg" />
+    <img src="https://img.shields.io/badge/Swift-5.2-orange.svg" />
     <a href="https://swift.org/package-manager">
         <img src="https://img.shields.io/badge/swiftpm-compatible-brightgreen.svg?style=flat" alt="Swift Package Manager" />
     </a>
@@ -228,6 +228,12 @@ try DeliciousRecipes().publish(using: [
 
 For a real-world example of a Publish plugin, check out the [official Splash plugin](https://github.com/johnsundell/splashpublishplugin), which makes it really easy to integrate the [Splash syntax highlighter](https://github.com/johnsundell/splash) with Publish.
 
+## System requirements
+
+To be able to successfully use Publish, make sure that your system has Swift version 5.2 (or later) installed. If you’re using a Mac, also make sure that `xcode-select` is pointed at an Xcode installation that includes the required version of Swift, and that you’re running macOS Catalina (10.15) or later. 
+
+Please note that Publish **does not** officially support any form of beta software, including beta versions of Xcode and macOS, or unreleased versions of Swift.
+
 ## Installation
 
 Publish is distributed using the [Swift Package Manager](https://swift.org/package-manager). To install it into a project, add it as a dependency within your `Package.swift` manifest: