readme: add badge

Author: Ark4ne

readme.md

@@ -4,6 +4,7 @@ JsonApi - Laravel Resource
 A Lightweight [{JSON:API}](https://jsonapi.org/) Resource for Laravel.
 
 [![Build Status](https://app.travis-ci.com/Ark4ne/laravel-json-api.svg?branch=master)](https://app.travis-ci.com/Ark4ne/laravel-json-api)
+[![codecov](https://codecov.io/gh/Ark4ne/laravel-json-api/branch/master/graph/badge.svg?token=F7XBLAGTDP)](https://codecov.io/gh/Ark4ne/laravel-json-api)
 
 # Installation
 ```shell