coverall badge in readme

Author: ernestoarbitrio

README.md

@@ -3,6 +3,7 @@ Genomic Data Commons API wrapper
 A simple Python wrapper for the [GDC Application Programming Interface (API)](https://portal.gdc.cancer.gov/)
 
 [![Build Status](https://travis-ci.com/histolab/gdc-api-wrapper.svg?branch=master)](https://travis-ci.com/histolab/gdc-api-wrapper)
+[![Coverage Status](https://coveralls.io/repos/github/histolab/gdc-api-wrapper/badge.svg?branch=master)](https://coveralls.io/github/histolab/gdc-api-wrapper?branch=master)
 
 The GDC API drives the GDC Data and Submission Portals and provides programmatic access to GDC functionality. This includes searching for, downloading, and submitting data and metadata.