Merge pull request #429 from RocketChat/update-readme-googleplay-link

[DOCS] Add GooglePlay link to README

README.md

+# 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)
 
-# Rocket.Chat.Android
-Rocket.Chat Native Android Application.
+# Get it from Google Play
+
+[![Rocket.Chat on Google Play](https://user-images.githubusercontent.com/551004/29770692-a20975c6-8bc6-11e7-8ab0-1cde275496e0.png)](https://play.google.com/store/apps/details?id=chat.rocket.android)
 
 ## How to build