add a contributor's guide

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
+[MPI-SWS GitLab](https://gitlab.mpi-sws.org/users/sign_in) (use the "Register"
+tab).
+
+To contribute code, please contact [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
+changes in your fork, and create a merge request.