Added code coverage badge (#21)

Author: Javier Morant

README.md

@@ -6,6 +6,7 @@ UI component for React Native that allows to create a listview with N levels of
 ![platforms](https://img.shields.io/badge/platforms-Android%20%7C%20iOS-brightgreen.svg?style=flat-square)
 [![Greenkeeper badge](https://badges.greenkeeper.io/fjmorant/react-native-nested-listview.svg)](https://greenkeeper.io/)
 [![CircleCI](https://circleci.com/gh/fjmorant/react-native-nested-listview.svg?style=shield)](https://circleci.com/gh/fjmorant/react-native-nested-listview)
+[![codecov](https://codecov.io/gh/fjmorant/react-native-nested-listview/branch/master/graph/badge.svg)](https://codecov.io/gh/fjmorant/react-native-nested-listview)
 [![npm](https://img.shields.io/npm/v/react-native-nested-listview.svg?style=flat-square)](https://www.npmjs.com/package/react-native-nested-listview)
 [![github release](https://img.shields.io/github/release/fjmorant/react-native-nested-listview.svg?style=flat-square)](https://github.com/fjmorant/react-native-nested-listview/releases)