Update README with new logo and Slack self-signup link.

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README.md

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-# ReactiveSwift
-
-#### Streams of values over time. Tailored for Swift.
-
+<p align="center">
+	<a href="https://github.com/ReactiveCocoa/ReactiveSwift/"><img src="Logo/PNG/logo-Swift.png" alt="ReactiveSwift" height="70" /></a><br /><br />
+	Streams of values over time. Tailored for Swift.<br /><br />
+	<a href="https://reactiveswift.herokuapp.com"><img src="Logo/PNG/JoinSlack.png" alt="Join the ReactiveSwift Slack community." height="40" /></a>
+</p>
+<br />
 [![Carthage compatible](https://img.shields.io/badge/Carthage-compatible-4BC51D.svg?style=flat)](#carthage) [![CocoaPods compatible](https://img.shields.io/cocoapods/v/ReactiveSwift.svg)](#cocoapods) [![SwiftPM compatible](https://img.shields.io/badge/SwiftPM-compatible-orange.svg)](#swift-package-manager) [![GitHub release](https://img.shields.io/github/release/ReactiveCocoa/ReactiveSwift.svg)](https://github.com/ReactiveCocoa/ReactiveSwift/releases) ![Swift 3.0.x](https://img.shields.io/badge/Swift-3.0.x-orange.svg) ![platforms](https://img.shields.io/badge/platform-iOS%20%7C%20macOS%20%7C%20tvOS%20%7C%20watchOS%20%7C%20Linux-lightgrey.svg)
 
 ☕️ [Looking for Cocoa extensions?][ReactiveCocoa]