chore: add diffpy user group to readme

Author: cadenmyers13

README.rst

@@ -119,6 +119,8 @@ You may consult our `online documentation <https://diffpy.github.io/diffpy.srrea
 Support and Contribute
 ----------------------
 
+`Diffpy user group <https://groups.google.com/g/diffpy-users>`_ is the discussion forum for general questions and discussions about the use of diffpy.srreal. Please join the diffpy.srreal users community by joining the Google group. The diffpy.srreal project welcomes your expertise and enthusiasm!
+
 If you see a bug or want to request a feature, please `report it as an issue <https://github.com/diffpy/diffpy.srreal/issues>`_ and/or `submit a fix as a PR <https://github.com/diffpy/diffpy.srreal/pulls>`_.
 
 Feel free to fork the project and contribute. To install diffpy.srreal