add Code Climate badge to README.md

Author: xalopp

README.md

@@ -5,6 +5,7 @@ Provides a PHP CodeSniffer ruleset for the MO4 coding standard
 [![Build Status](https://travis-ci.org/mayflower/mo4-coding-standard.svg?branch=master)](https://travis-ci.org/mayflower/mo4-coding-standard)
 [![Build Status](https://codecov.io/gh/mayflower/mo4-coding-standard/branch/master/graph/badge.svg)](https://codecov.io/gh/mayflower/mo4-coding-standard/branch/master/)
 [![Scrutinizer Quality Level](https://scrutinizer-ci.com/g/mayflower/mo4-coding-standard/badges/quality-score.png?b=master)](https://scrutinizer-ci.com/g/mayflower/mo4-coding-standard)
+[![Maintainability](https://api.codeclimate.com/v1/badges/16114548a0315d993868/maintainability)](https://codeclimate.com/github/mayflower/mo4-coding-standard/maintainability)
 
 [![Latest Stable Version](https://poser.pugx.org/mayflower/mo4-coding-standard/v/stable)](https://packagist.org/packages/mayflower/mo4-coding-standard)
 [![Total Downloads](https://poser.pugx.org/mayflower/mo4-coding-standard/downloads)](https://packagist.org/packages/mayflower/mo4-coding-standard)