add code cov to readme

Author: Remi-Gau

README.md

@@ -3,6 +3,8 @@
 
 [![Build Status](https://travis-ci.com/cpp-lln-lab/CPP_PTB.svg?branch=master)](https://travis-ci.com/cpp-lln-lab/CPP_PTB)
 
+[![codecov](https://codecov.io/gh/cpp-lln-lab/CPP_PTB/branch/master/graph/badge.svg)](https://codecov.io/gh/cpp-lln-lab/CPP_PTB)
+
 <!-- ALL-CONTRIBUTORS-BADGE:START - Do not remove or modify this section -->
 [![All Contributors](https://img.shields.io/badge/all_contributors-3-orange.svg?style=flat-square)](#contributors-)
 <!-- ALL-CONTRIBUTORS-BADGE:END -->