Add codecov badge

Author: Art4

README.md

@@ -3,6 +3,7 @@
 [![Latest Version](https://img.shields.io/github/release/Art4/json-api-client.svg)](https://github.com/Art4/json-api-client/releases)
 [![Software License](https://img.shields.io/badge/license-GPL3-brightgreen.svg)](LICENSE)
 [![Build Status](https://github.com/art4/json-api-client/actions/workflows/unit-tests.yml/badge.svg?branch=v1.x)](https://github.com/Art4/json-api-client/actions)
+[![codecov](https://codecov.io/gh/Art4/json-api-client/graph/badge.svg?token=5oNNDEUWgZ)](https://codecov.io/gh/Art4/json-api-client)
 [![Total Downloads](https://img.shields.io/packagist/dt/art4/json-api-client.svg)](https://packagist.org/packages/art4/json-api-client)
 
 JsonApiClient :construction_worker_woman: is a PHP Library to validate and handle the response body from a [JSON API](http://jsonapi.org) Server.