update CONTRIBUTING

CONTRIBUTING.md

 # CONTRIBUTING TO THE IRIS COQ DEVELOPMENT
 
 Discussion about the Iris Coq development happens on the mailing list
-[iris-club@lists.mpi-sws.org](https://lists.mpi-sws.org/listinfo/iris-club).
-This is also the right place to ask questions.
-
-If you want to report a bug, please use the
-[issue tracker](https://gitlab.mpi-sws.org/FP/iris-coq/issues).  You will have
-to create an account at the
+[iris-club@lists.mpi-sws.org](https://lists.mpi-sws.org/listinfo/iris-club) and
+in the [Iris Chat](https://mattermost.mpi-sws.org/iris).  This is also the right
+place to ask questions.  The chat requires an account at the
 [MPI-SWS GitLab](https://gitlab.mpi-sws.org/users/sign_in) (use the "Register"
 tab).
 
-To contribute code, please send your MPI-SWS GitLab username to
+If you want to report a bug, please use the
+[issue tracker](https://gitlab.mpi-sws.org/FP/iris-coq/issues).  To contribute
+code, please send your MPI-SWS GitLab username to
 [Ralf Jung](https://gitlab.mpi-sws.org/jung) to enable personal projects for
 your account.  Then you can fork the
 [Iris git repository](https://gitlab.mpi-sws.org/FP/iris-coq/), make your