Update readme with Carthage instructions.

Author: ole

README.md

@@ -12,11 +12,17 @@ For more info, see my accompanying [blog article](https://oleb.net/blog/2017/02/
 
 [![Build Status](https://travis-ci.org/ole/SortedArray.svg?branch=master)](https://travis-ci.org/ole/SortedArray)
 
+## Supported Platforms
+
+Since the code has no dependencies other than the Swift standard library (it doesn't even use Foundation), it should work on all platforms where Swift is available.
+
+I tested it on macOS, iOS, tvOS, and Linux.
+
 ## Usage
 
-Clone the repository and add or copy `SortedArray.swift` to your project. It has no dependencies.
+### Swift Package Manager
 
-If you want to try this out in a Swift Package Manager project, add this to your `Package.swift` file:
+Add this to your `Package.swift` file:
 
 ```swift
 // Package.swift
@@ -30,6 +36,20 @@ let package = Package(
 )
 ```
 
+### Carthage
+
+Add this to your `Cartfile`:
+
+```
+github "ole/SortedArray" ~> 0.3
+```
+
+Integration via Carthage should work for macOS, iOS, tvOS, and watchOS targets.
+
+### Manually
+
+Clone the repository and add or copy `SortedArray.swift` to your project. It has no dependencies.
+
 ## Dependencies
 
 None.