remove ref to the mailing list (#453)

README.md

@@ -178,10 +178,6 @@ status as a core developer.
 
 ## Getting help
 
-Join the mailing list to ask for help or stay up-to-date:
-
-https://groups.google.com/forum/#!forum/maestro-help
-
 For answers to frequently asked questions and solutions to common
 problems, consult the [FAQ
 section](https://amrex-astro.github.io/MAESTROeX/docs/faq.html) of the
@@ -190,3 +186,9 @@ User's Guide.
 If you discover any bugs in the code or would like to request further
 help, then please submit a issue to the MAESTROeX Github repository
 describing your problem.
+
+For general questions, post to the discussion forum:
+https://github.com/AMReX-Astro/MAESTROeX/discussions
+
+and for problems, raise an issue:
+https://github.com/AMReX-Astro/MAESTROeX/issues