add npm downloads badge

Author: slorber

README.md

@@ -1,6 +1,5 @@
 # React Navigation Hooks
-
-[![npm version](https://badge.fury.io/js/react-navigation-hooks.svg)](https://badge.fury.io/js/react-navigation-hooks) [![CircleCI badge](https://circleci.com/gh/react-navigation/hooks/tree/master.svg?style=shield)](https://circleci.com/gh/react-navigation/hooks/tree/master) [![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg)](https://reactnavigation.org/docs/contributing.html)
+[![npm version](https://badge.fury.io/js/react-navigation-hooks.svg)](https://badge.fury.io/js/react-navigation-hooks) [![npm downloads](https://img.shields.io/npm/dm/react-navigation-hooks.svg)](https://www.npmjs.com/package/react-navigation-hooks) [![CircleCI badge](https://circleci.com/gh/react-navigation/hooks/tree/master.svg?style=shield)](https://circleci.com/gh/react-navigation/hooks/tree/master) [![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg)](https://reactnavigation.org/docs/contributing.html)
 
 🏄‍♀️ Surfing the wave of React Hook hype with a few convenience hooks for `@react-navigation/core` v3. Destined to work on web, server, and React Native. Contributions welcome!