Add downloads badge

Author: BenMorel

README.md

@@ -10,6 +10,7 @@ but may be used with other vocabularies.
 [![Build Status](https://github.com/brick/structured-data/workflows/CI/badge.svg)](https://github.com/brick/structured-data/actions)
 [![Coverage Status](https://coveralls.io/repos/github/brick/structured-data/badge.svg?branch=master)](https://coveralls.io/github/brick/structured-data?branch=master)
 [![Latest Stable Version](https://poser.pugx.org/brick/structured-data/v/stable)](https://packagist.org/packages/brick/structured-data)
+[![Total Downloads](https://poser.pugx.org/brick/structured-data/downloads)](https://packagist.org/packages/brick/structured-data)
 [![License](https://img.shields.io/badge/license-MIT-blue.svg)](http://opensource.org/licenses/MIT)
 
 ### Installation