added coveralls badge

Author: adamdubiel

README.md

@@ -4,6 +4,7 @@ axion-release-plugin
 *gradle release and version management plugin*
 
 [![Build Status](https://travis-ci.org/allegro/axion-release-plugin.svg?branch=master)](https://travis-ci.org/allegro/axion-release-plugin)
+[![Coverage Status](https://coveralls.io/repos/allegro/axion-release-plugin/badge.svg?branch=development)](https://coveralls.io/r/allegro/axion-release-plugin)
 [![readthedocs](https://readthedocs.org/projects/axion-release-plugin/badge/?version=latest) ](http://axion-release-plugin.readthedocs.org/en/latest/)
 [![Maven Central](https://maven-badges.herokuapp.com/maven-central/pl.allegro.tech.build/axion-release-plugin/badge.svg?style=flat)](https://maven-badges.herokuapp.com/maven-central/pl.allegro.tech.build/axion-release-plugin)
 [![JCenter](https://api.bintray.com/packages/allegro/maven/axion-release-plugin/images/download.svg) ](https://bintray.com/allegro/maven/axion-release-plugin/_latestVersion)