Add a Gitter chat badge to README.md (OSGeo#2540)

* Add Gitter badge

README.md

@@ -6,6 +6,7 @@
 [![OSGeo4W](https://github.com/OSGeo/grass/workflows/OSGeo4W/badge.svg)](https://github.com/OSGeo/grass/actions?query=workflow%3AOSGeo4W)
 [![CentOS](https://github.com/OSGeo/grass/workflows/CentOS/badge.svg)](https://github.com/OSGeo/grass/actions?query=workflow%3ACentOS)
 [![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)
 
 # GRASS GIS Repository
 
@@ -20,7 +21,6 @@ Launch this repository in Binder and experiment with GRASS's Python API in Jupyt
 
 [![Binder](https://camo.githubusercontent.com/581c077bdbc6ca6899c86d0acc6145ae85e9d80e6f805a1071793dbe48917982/68747470733a2f2f6d7962696e6465722e6f72672f62616467655f6c6f676f2e737667)](https://mybinder.org/v2/gh/OSGeo/grass/main?urlpath=lab%2Ftree%2Fdoc%2Fnotebooks%2Fjupyter_example.ipynb)
 
-
 ## How to get write access here
 
 In general: you don't really need write access as you can simply open