docs(readme): add coverage badge

Author: Kamil Kisiela

README.md

@@ -3,6 +3,7 @@ FormlyMaterial
 
 [![GitHub version](https://badge.fury.io/gh/wieldo%2Fangular-formly-templates-material.svg)](https://badge.fury.io/gh/wieldo%2Fangular-formly-templates-material)
 [![Build Status](https://travis-ci.org/wieldo/angular-formly-templates-material.svg)](https://travis-ci.org/wieldo/angular-formly-templates-material)
+[![Coverage Status](https://coveralls.io/repos/wieldo/angular-formly-templates-material/badge.svg?branch=master&service=github)](https://coveralls.io/github/wieldo/angular-formly-templates-material?branch=master)
 [![Codacy Badge](https://api.codacy.com/project/badge/grade/a2cd4c7c2d74467281e309a65be49e8f)](https://www.codacy.com/app/mys-sterowiec/angular-formly-templates-material)
 
 Material Design Templates for [Angular-Formly](http://angular-formly.com). Modern & flexible forms configured easily in a JSON object.