add citation file, doi badge

Author: nschloe

CITATION.cff

@@ -0,0 +1,10 @@
+cff-version: 1.2.0
+message: "If you use this software, please cite it as below."
+authors:
+- family-names: "Schlömer"
+  given-names: "Nico"
+  orcid: "https://orcid.org/0000-0001-5228-0946"
+title: "pygalmesh: Python interface for CGAL's meshing tools"
+doi: 10.5281/zenodo.5564818
+url: https://github.com/nschloe/pygalmesh
+license: GPL-3.0

README.md

@@ -7,6 +7,7 @@
 [![Anaconda Cloud](https://anaconda.org/conda-forge/pygalmesh/badges/version.svg?=style=flat-square)](https://anaconda.org/conda-forge/pygalmesh/)
 [![Packaging status](https://repology.org/badge/tiny-repos/pygalmesh.svg)](https://repology.org/project/pygalmesh/versions)
 [![PyPI pyversions](https://img.shields.io/pypi/pyversions/pygalmesh.svg?style=flat-square)](https://pypi.org/pypi/pygalmesh/)
+[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.5564818.svg?style=flat-square)](https://doi.org/10.5281/zenodo.5564818)
 [![GitHub stars](https://img.shields.io/github/stars/nschloe/pygalmesh.svg?style=flat-square&label=Stars&logo=github)](https://github.com/nschloe/pygalmesh)
 [![PyPi downloads](https://img.shields.io/pypi/dm/pygalmesh.svg?style=flat-square)](https://pypistats.org/packages/pygalmesh)