Update documentation.

Author: objecthub

Cartfile

@@ -1 +1 @@
-github "objecthub/swift-numberkit"
+github "objecthub/swift-numberkit" ~> 1.6

README.md

@@ -2,10 +2,11 @@
 ======================================================
 
 <p>
-<a href="https://developer.apple.com/osx/"><img src="https://img.shields.io/badge/Platform-macOS-blue.svg?style=flat" alt="Platform: macOS" /></a>
-<a href="https://developer.apple.com/swift/"><img src="https://img.shields.io/badge/Language-Swift%203.1-green.svg?style=flat" alt="Language: Swift 3.1" /></a>
-<a href="https://developer.apple.com/xcode/"><img src="https://img.shields.io/badge/IDE-Xcode%208.3-orange.svg?style=flat" alt="IDE: Xcode 8.3" /></a>
-<a href="https://raw.githubusercontent.com/objecthub/swift-lispkit/master/LICENSE"><img src="http://img.shields.io/badge/License-Apache-lightgrey.svg?style=flat" alt="License: Apache" /></a>
+[![Platform: macOS](https://img.shields.io/badge/Platform-macOS-blue.svg?style=flat)](https://developer.apple.com/osx/)]
+[![Language: Swift 3.1](https://img.shields.io/badge/Language-Swift%203.1-green.svg?style=flat)](https://developer.apple.com/swift/)
+[![IDE: Xcode 8.3](https://img.shields.io/badge/IDE-Xcode%208.3-orange.svg?style=flat)](https://developer.apple.com/xcode/)
+[![Carthage: compatible](https://img.shields.io/badge/Carthage-compatible-4BC51D.svg?style=flat)](https://github.com/Carthage/Carthage)
+[![License: Apache](http://img.shields.io/badge/License-Apache-lightgrey.svg?style=flat)](https://raw.githubusercontent.com/objecthub/swift-lispkit/master/LICENSE)
 </p>