add doi badge

Author: Remi-Gau

README.md

@@ -1,10 +1,18 @@
+**Cite it**
+
+[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.4007672.svg)](https://doi.org/10.5281/zenodo.4007672)
+
+**Unit tests and coverage**
+
 [![](https://img.shields.io/badge/Octave-CI-blue?logo=Octave&logoColor=white)](https://github.com/cpp-lln-lab/CPP_PTB/actions)
 ![](https://github.com/cpp-lln-lab/CPP_PTB/workflows/CI/badge.svg)
 
 [![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)
 
+**Contributors**
+
 <!-- 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-)