docs: add coverity badge to README.md (#3630)

Add Coverity badge to README.md. This would make it easier for the average contributor (and the public as well) to see that GRASS is  using the Coverity platform for static analysis.

README.md

@@ -8,6 +8,7 @@
 [![OSGeo4W](https://github.com/OSGeo/grass/workflows/OSGeo4W/badge.svg)](https://github.com/OSGeo/grass/actions?query=workflow%3AOSGeo4W)
 [![OpenSSF Scorecard](https://api.securityscorecards.dev/projects/github.com/OSGeo/grass/badge)](https://securityscorecards.dev/viewer/?uri=github.com/OSGeo/grass)
 [![OpenSSF Best Practices](https://www.bestpractices.dev/projects/2470/badge)](https://www.bestpractices.dev/projects/2470)
+[![Coverity](https://scan.coverity.com/projects/1038/badge.svg)](https://scan.coverity.com/projects/grass)
 [![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.5176030.svg)](https://doi.org/10.5281/zenodo.5176030)
 [![Join the chat at https://gitter.im/grassgis/community](https://badges.gitter.im/grassgis/community.svg)](https://gitter.im/grassgis/community?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)