Updated description and instructions for Carthage

I felt the description and instructions for *Carthage* weren't accurate.

- reworded the introduction to *Carthage* to be closer to how it describes itself.
- added project integration step instruction for *Carthage* based on its documentation.

README.md

@@ -74,7 +74,7 @@ $ pod install
 
 ### Carthage
 
-[Carthage](https://github.com/Carthage/Carthage) is a decentralized dependency manager that automates the process of adding frameworks to your Cocoa application.
+[Carthage](https://github.com/Carthage/Carthage) is a decentralized dependency manager that builds your dependencies and provides you with binary frameworks.
 
 You can install Carthage with [Homebrew](http://brew.sh/) using the following command:
 
@@ -89,6 +89,8 @@ To integrate Alamofire into your Xcode project using Carthage, specify it in you
 github "Alamofire/Alamofire" ~> 3.0
 ```
 
+Run `carthage` to build the framework and drag the built `Alamofire.framework` into your Xcode project.
+
 ### Manually
 
 If you prefer not to use either of the aforementioned dependency managers, you can integrate Alamofire into your project manually.