README.md: add coveralls badge [ci skip]

README.md

@@ -11,6 +11,7 @@ GDAL - Geospatial Data Abstraction Library
 [![Build Status](https://scan.coverity.com/projects/749/badge.svg?flat=1)](https://scan.coverity.com/projects/gdal)
 [![Documentation build Status](https://github.com/OSGeo/gdal/workflows/Docs/badge.svg)](https://github.com/osgeo/gdal/actions?query=workflow%3A%22Docs%22+branch%3Amaster)
 [![Fuzzing Status](https://oss-fuzz-build-logs.storage.googleapis.com/badges/gdal.svg)](https://bugs.chromium.org/p/oss-fuzz/issues/list?sort=-opened&can=1&q=proj:gdal)
+[![Coverage Status](https://coveralls.io/repos/github/OSGeo/gdal/badge.svg?branch=master)](https://coveralls.io/github/OSGeo/gdal?branch=master)
 
 [![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.5884351.svg)](https://doi.org/10.5281/zenodo.5884351)