Added zenodo badge

Author: Sylvain MARIE

docs/index.md

@@ -5,6 +5,7 @@
 [![Python versions](https://img.shields.io/pypi/pyversions/pytest-cases.svg)](https://pypi.python.org/pypi/pytest-cases/) [![Build Status](https://travis-ci.org/smarie/python-pytest-cases.svg?branch=master)](https://travis-ci.org/smarie/python-pytest-cases) [![Tests Status](https://smarie.github.io/python-pytest-cases/junit/junit-badge.svg?dummy=8484744)](https://smarie.github.io/python-pytest-cases/junit/report.html) [![codecov](https://codecov.io/gh/smarie/python-pytest-cases/branch/master/graph/badge.svg)](https://codecov.io/gh/smarie/python-pytest-cases)
 
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+[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.3937830.svg)](https://doi.org/10.5281/zenodo.3937830)
 
 !!! success "Brand new v2, [check the changes](./changelog.md#200---less-boilerplate-and-full-pytest-alignment-unlocking-advanced-features) !"