Add Carthage documentation

README.md

@@ -1,4 +1,4 @@
-[![Gitter chat](http://img.shields.io/badge/gitter-join%20chat%20%E2%86%92-brightgreen.svg)](https://gitter.im/opentracing/public) [![Version](https://img.shields.io/cocoapods/v/opentracing.svg?style=flat)](http://cocoapods.org/pods/opentracing) [![License](https://img.shields.io/cocoapods/l/opentracing.svg?style=flat)](http://cocoapods.org/pods/opentracing) [![Platform](https://img.shields.io/cocoapods/p/opentracing.svg?style=flat)](http://cocoapods.org/pods/opentracing)
+[![Gitter chat](http://img.shields.io/badge/gitter-join%20chat%20%E2%86%92-brightgreen.svg)](https://gitter.im/opentracing/public) [![Version](https://img.shields.io/cocoapods/v/opentracing.svg?style=flat)](http://cocoapods.org/pods/opentracing) [![License](https://img.shields.io/cocoapods/l/opentracing.svg?style=flat)](http://cocoapods.org/pods/opentracing) [![Platform](https://img.shields.io/cocoapods/p/opentracing.svg?style=flat)](http://cocoapods.org/pods/opentracing) [![Carthage Compatible](https://img.shields.io/badge/Carthage-compatible-4BC51D.svg?style=flat)](https://github.com/Carthage/Carthage)
 
 # OpenTracing API for Objective-C
 
@@ -17,3 +17,12 @@ Use [CocoaPods](https://guides.cocoapods.org/using/getting-started.html)!
 ```ruby
 pod "opentracing"
 ```
+
+## Installation (Carthage)
+
+Use [Carthage](https://github.com/Carthage/Carthage#quick-start)!
+
+```ruby
+github "undefinedlabs/opentracing-objc"
+```
+