add coverage badge

Author: nekufa

README.md

@@ -4,6 +4,7 @@
 [![Latest Version](https://img.shields.io/github/release/tarantool-php/symfony-lock.svg?style=flat-square)](https://github.com/tarantool-php/symfony-lock/releases)
 [![Total Downloads](https://img.shields.io/packagist/dt/tarantool/symfony-lock.svg?style=flat-square)](https://packagist.org/packages/tarantool/symfony-lock)
 [![Scrutinizer Code Quality](https://scrutinizer-ci.com/g/tarantool-php/symfony-lock/badges/quality-score.png?b=master)](https://scrutinizer-ci.com/g/tarantool-php/symfony-lock/?branch=master)
+[![Code Coverage](https://scrutinizer-ci.com/g/tarantool-php/mapper/badges/coverage.png?b=master)](https://scrutinizer-ci.com/g/tarantool-php/mapper/?branch=master)
 [![Telegram](https://img.shields.io/badge/Telegram-join%20chat-blue.svg)](https://t.me/tarantool_php)
 
 # About