Add a contributor's guide

Based on the one of Ralf Jung for Iris.

CONTRIBUTING.md

+# CONTRIBUTING TO THE COQ-STD++ DEVELOPMENT
+
+If you want to report a bug, please use the
+[issue tracker](https://gitlab.mpi-sws.org/robbertkrebbers/coq-stdpp/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
+[Coq-std++ git repository](https://gitlab.mpi-sws.org/robbertkrebbers/coq-stdpp),
+make your changes in your fork, and create a merge request.
+