readme: add badge for downloads

Author: mfn

README.md

@@ -3,6 +3,7 @@
 [![Latest Stable Version](https://poser.pugx.org/rebing/graphql-laravel/v/stable)](https://packagist.org/packages/rebing/graphql-laravel)
 [![License](https://poser.pugx.org/rebing/graphql-laravel/license)](https://packagist.org/packages/rebing/graphql-laravel)
 [![Tests](https://github.com/rebing/graphql-laravel/workflows/Tests/badge.svg)](https://github.com/rebing/graphql-laravel/actions?query=workflow%3ATests)
+[![Downloads](https://img.shields.io/packagist/dt/rebing/graphql-laravel.svg?style=flat-square)](https://packagist.org/packages/rebing/graphql-laravel)
 [![Get on Slack](https://img.shields.io/badge/slack-join-orange.svg)](https://join.slack.com/t/rebing-graphql/shared_invite/enQtNTE5NjQzNDI5MzQ4LTdhNjk0ZGY1N2U1YjE4MGVlYmM2YTc2YjQ0MmIwODY5MWMwZWIwYmY1MWY4NTZjY2Q5MzdmM2Q3NTEyNDYzZjc)
 
 Uses Facebook GraphQL with Laravel 6.0+. It is based on the PHP implementation [here](https://github.com/webonyx/graphql-php). You can find more information about GraphQL in the [GraphQL Introduction](http://facebook.github.io/react/blog/2015/05/01/graphql-introduction.html) on the [React](http://facebook.github.io/react) blog or you can read the [GraphQL specifications](https://facebook.github.io/graphql/). This is a work in progress.