README: add GMD badge

Author: MuellerSeb

README.md

@@ -1,5 +1,5 @@
 # Welcome to GSTools
-
+[![GMD](https://img.shields.io/badge/GMD-10.5194%2Fgmd--15--3161--2022-orange)](https://doi.org/10.5194/gmd-15-3161-2022)
 [![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.1313628.svg)](https://doi.org/10.5281/zenodo.1313628)
 [![PyPI version](https://badge.fury.io/py/gstools.svg)](https://badge.fury.io/py/gstools)
 [![Conda Version](https://img.shields.io/conda/vn/conda-forge/gstools.svg)](https://anaconda.org/conda-forge/gstools)