Updated README sample code to 3.0 APIs and added a migration guide.

Documentation/Alamofire 3.0 Migration Guide.md

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+# Alamofire 3.0 Migration Guide
+
+Alamofire 3.0 is the latest major release of Alamofire, an HTTP networking library for iOS, Mac OS X and watchOS written in Swift. As a major release, following Semantic Versioning conventions, 3.0 introduces several API-breaking changes that one should be aware of.
+
+This guide is provided in order to ease the transition of existing applications using Alamofire 2.x to the latest APIs, as well as explain the design and structure of new and changed functionality.
+
+## Requirements
+
+Alamofire 3.0 officially supports iOS 8+, Mac OS X 10.9+, watchOS 2.0, Xcode 7 and Swift 2.0. If you'd like to use Alamofire in a project targeting iOS 7 and Swift 1.x, use the latest tagged 1.x release.
+
+## Reasons for Bumping to 3.0
+
+TODO
+
+## Benefits of Upgrading
+
+TODO
+
+---
+
+## Breaking API Changes
+
+TODO
+
+### Result Type
+
+TODO
+
+### Response
+
+TODO
+
+### Response Serializers
+
+TODO
+
+#### Response Serializer Type
+
+TODO
+
+---
+
+## New Features
+
+### Dependency Injection
+
+TODO
+
+#### Session Delegate
+
+TODO
+
+#### URL Session 
+
+TODO