Bumped the required version of Xcode to beta 6 in the requirements section.

README.md

@@ -32,7 +32,7 @@ In order to keep Alamofire focused specifically on core networking implementatio
 ## Requirements
 
 - iOS 9.0+ / Mac OS X 10.11+ / tvOS 9.0+ / watchOS 2.0+
-- Xcode 8.0 beta 3+
+- Xcode 8.0 beta 6+
 
 ## Migration Guides