Add required attribution text to LICENSE

Consuming GSKeychain via CocoaPods is very easy, but the default Acknowledgements.plist built by CocoaPods includes only the pod name followed by the contents of the LICENSE file. Putting the required attribution text in the LICENSE file makes it so that consuming apps need not do anything beyond including this auto-generated Acknowledgements.plist [in their settings bundle](https://github.com/CocoaPods/CocoaPods/wiki/Acknowledgements#ios-setting-bundle).