Add very basic instructions and a demo link to the README

Author: jspahrsummers

README.md

@@ -1,5 +1,15 @@
 # ReactiveAnimation [![Carthage compatible](https://img.shields.io/badge/Carthage-compatible-4BC51D.svg?style=flat)](https://github.com/Carthage/Carthage)
 
-Easy animations using ReactiveCocoa signals.
+Easy animations using ReactiveCocoa signals. See the [Mac
+demo](https://github.com/ReactiveCocoa/ReactiveAnimation-MacDemo) for an example
+of how the library works.
 
-Released under the [MIT license](LICENSE.md).
+ReactiveAnimation is released under the [MIT license](LICENSE.md).
+
+### Getting Started
+
+To import the ReactiveAnimation framework, add `ReactiveAnimation.xcodeproj` as
+a subproject and add the appropriate framework target to your application.
+
+Alternatively, you can use [Carthage](https://github.com/Carthage/Carthage) to
+build the framework for you.