Add package manager instructions

Author: CodaFi

Package.swift

@@ -1,11 +1,13 @@
 import PackageDescription
 
 let package = Package(
-  name: "Concurrent",
-  targets: [
-    Target(
-      name: "Concurrent",
-      dependencies: []),
+	name: "Concurrent",
+	targets: [
+		Target(
+			name: "Concurrent",
+			dependencies: []),
   ]
 )
 
+let libConcurrent = Product(name: "Concurrent", type: .Library(.Dynamic), modules: "Concurrent")
+products.append(libConcurrent)

README.md

@@ -139,6 +139,14 @@ Concurrent supports OS X 10.9+ and iOS 7.0+.
 Installation
 =====
 
+#### Swift Package Manager
+
+- Add SwiftCheck to your `Package.swift` file's dependencies section:
+
+```
+.Package(url: "https://github.com/typelift/Concurrent.git", versions: Version(0,4,0)..<Version(1,0,0))
+```
+
 #### Carthage
 Create a `Cartfile` that lists the framework and run `carthage bootstrap`. Follow the [instructions](https://github.com/Carthage/Carthage#if-youre-building-for-ios) to add `$(SRCROOT)/Carthage/Build/iOS/Concurrent.framework` to an iOS project.
 
@@ -147,7 +155,7 @@ github "typelift/Concurrent"
 ```
 
 #### Manually
-1. Download and drop ```/Sources``` folder in your project.  
+1. Download and drop `/Sources` folder in your project.  
 2. Congratulations!  
 
 #### Framework