Updates Supporting Swift 2.3 and Swift 3 section

Updates `Supporting Swift 2.3 and Swift 3` section with examples for Carthage

README.md

@@ -17,10 +17,19 @@ To install **Reachability.swift** for Swift 2.3 using CocoaPods, include the fol
 pod 'ReachabilitySwift', '~> 2.4'
 ```
 
+To install **Reachability.swift** for Swift 2.3 using Carthage, include the following in your Cartfile
+```
+github "ashleymills/Reachability.swift" ~> 2.3
+```
+
 To install **Reachability.swift** for Swift 3.x using CocoaPods, include the following in your Podfile
 ```
 pod 'ReachabilitySwift', '~> 3'
 ```
+To install **Reachability.swift** for Swift 3.x using Carthage, include the following in your Cartfile
+```
+github "ashleymills/Reachability.swift" ~> 3.0
+```
 ##Swift 3 breaking changes#
 
 Previously: