Adding the CodeCov results to README.md
This change adds the code coverage results of develop branch to README.md, which will let the visitors and the current developers see the results directly at the repo's homepage!
Thanks!
| ------------------ |
| You **cannot** use this library for Android app development. Have a look at [kaaes/spotify-web-api-android](https://www.github.com/kaaes/spotify-web-api-android) and [Spotify's Android SDK](https://developer.spotify.com/technologies/spotify-android-sdk/) and [see why](https://github.com/thelinmichael/spotify-web-api-java/issues/120#issuecomment-363266591). |
-Spotify Web API Java [![Build Status](https://travis-ci.org/thelinmichael/spotify-web-api-java.svg?branch=master)](https://travis-ci.org/thelinmichael/spotify-web-api-java)
+Spotify Web API Java [![Build Status](https://travis-ci.org/thelinmichael/spotify-web-api-java.svg?branch=master)](https://travis-ci.org/thelinmichael/spotify-web-api-java) [![codecov](https://codecov.io/gh/thelinmichael/spotify-web-api-java/branch/develop/graph/badge.svg)](https://codecov.io/gh/thelinmichael/spotify-web-api-java)
====================
This is a Java wrapper/client for the [Spotify Web API](https://developer.spotify.com/web-api/).
