Update README.md

Carthage compatibility added to readme.

Author: volkanbicer

README.md

@@ -1,6 +1,7 @@
 [![CocoaPods](https://img.shields.io/cocoapods/v/Iterable-iOS-SDK.svg?style=flat)](https://cocoapods.org/pods/Iterable-iOS-SDK)
 [![License](https://img.shields.io/cocoapods/l/Iterable-iOS-SDK.svg?style=flat)](https://opensource.org/licenses/MIT)
 [![Build Status](https://travis-ci.com/Iterable/swift-sdk.svg?branch=master)](https://travis-ci.com/Iterable/swift-sdk)
+[![Carthage compatible](https://img.shields.io/badge/Carthage-compatible-4BC51D.svg?style=flat)](https://github.com/Carthage/Carthage)
 
 # Iterable iOS SDK