feat(README): Add coverage badge

Author: ArnaudSpanneut

README.md

@@ -1,6 +1,7 @@
 # angular-sql-query
 
 [![CircleCI](https://circleci.com/gh/SimpliField/angular-sql-query.svg?style=svg)](https://circleci.com/gh/SimpliField/angular-sql-query)
+[![Coverage Status](https://coveralls.io/repos/github/SimpliField/angular-sql-query/badge.svg?branch=master)](https://coveralls.io/github/SimpliField/angular-sql-query?branch=master)
 [![Codacy Badge](https://api.codacy.com/project/badge/Grade/67af78db2da041f5a2d0dd9ca52850dd)](https://www.codacy.com?utm_source=github.com&utm_medium=referral&utm_content=SimpliField/sf-frontend&utm_campaign=Badge_Grade)
 [![Codacy Badge](https://api.codacy.com/project/badge/Coverage/67af78db2da041f5a2d0dd9ca52850dd)](https://www.codacy.com?utm_source=github.com&utm_medium=referral&utm_content=SimpliField/sf-frontend&utm_campaign=Badge_Coverage)