Update README.md

Add downloads per week

docs/README.md

@@ -1,3 +1,4 @@
+![npm](https://img.shields.io/npm/dw/react-native-navigation.svg)
 [![npm (tag)](https://img.shields.io/npm/v/react-native-navigation/alpha.svg)](https://github.com/wix/react-native-navigation/tree/v2#react-native-navigation-v2-wip)
 [![Build Status](https://jenkins-oss.wixpress.com/job/react-native-navigation-master/badge/icon)](https://jenkins-oss.wixpress.com/job/react-native-navigation-master/)
 [![Join us on Discord](https://img.shields.io/badge/discord-react--native--navigation-738bd7.svg?style=flat)](https://discord.gg/DhkZjq2)