Update readme

README.md

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-# iOS/Swift helper functions for storing text in Keychain
+# Helper functions for storing text in Keychain for iOS, OS X and WatchOS
 
 [![Carthage compatible](https://img.shields.io/badge/Carthage-compatible-4BC51D.svg?style=flat)][carthage]
 [![CocoaPods Version](https://img.shields.io/cocoapods/v/KeychainSwift.svg?style=flat)][cocoadocs]