Add note to README about adding Runes

If users want to use functional operators, they will need to link Runes
into their project as well.

Additionally, we should link directly to Carthage's installation
instructions, since they are currently in flux.

README.md

@@ -17,8 +17,18 @@ Add the following to your Cartfile:
 github "thoughtbot/Argo"
 ```
 
-Then run `carthage update`. Drag `Argo.framework` into your project, and
-you're good to go.
+Then run `carthage update`.
+
+Follow the current instructions in [Carthage's README][carthage-installation]
+for up to date installation instructions.
+
+[carthage-installation]: https://github.com/Carthage/Carthage#adding-frameworks-to-an-application
+
+If you want to use the functional operators (`<^>`, `<*>`, `>>-`) in your app,
+you'll also need to add `Runes.framework` to your project. [Runes] is a
+dependency of Argo, so you don't need to specify it in your Cartfile.
+
+[Runes]: https://github.com/thoughtbot/runes
 
 ### [CocoaPods]