Add logo to readme (#802)

.npmignore

 crashlytics-build.properties
 fabric.properties
 
+./logo.png
+
 
 ############
 # iOS

README.md

-# React Native Navigation
+
+<h1 align="center">
+  <img src="./logo.png"/><br>
+  React Native Navigation
+</h1>
 
 [![NPM Version](https://img.shields.io/npm/v/react-native-navigation.svg?style=flat)](https://www.npmjs.com/package/react-native-navigation)
 [![NPM Downloads](https://img.shields.io/npm/dm/react-native-navigation.svg?style=flat)](https://www.npmjs.com/package/react-native-navigation)
 [![Build Status](https://travis-ci.org/wix/react-native-navigation.svg?branch=master)](https://travis-ci.org/wix/react-native-navigation)
 [![Join us on Discord](https://img.shields.io/badge/discord-react--native--navigation-738bd7.svg?style=flat)](https://discord.gg/DhkZjq2)
 
-
 App-wide support for 100% native navigation with an easy cross-platform interface. For iOS, this package is a wrapper around [react-native-controllers](https://github.com/wix/react-native-controllers), but provides a simplified more abstract API over it. This abstract API will be unified with the Android solution which is currently work in progress. It also fully supports redux if you use it.
 
 <img src="https://github.com/wix/react-native/blob/master/assets/themes/bootstrap-3/images/demo.gif?raw=true" width="240">