adds rocket chat logo

README.md

+![Rocket.Chat logo](https://raw.githubusercontent.com/RocketChat/Rocket.Chat.Artwork/master/Logos/logo-dark.svg?sanitize=true)
+
 # Rocket.Chat Android native application
 
 [![CircleCI](https://circleci.com/gh/RocketChat/Rocket.Chat.Android/tree/develop.svg?style=shield)](https://circleci.com/gh/RocketChat/Rocket.Chat.Android/tree/develop) [![Build Status](https://travis-ci.org/RocketChat/Rocket.Chat.Android.svg?branch=develop)](https://travis-ci.org/RocketChat/Rocket.Chat.Android) [![Codacy Badge](https://api.codacy.com/project/badge/Grade/a81156a8682e4649994270d3670c3c83)](https://www.codacy.com/app/matheusjardimb/Rocket.Chat.Android)