Add CONTRIBUTING

CONTRIBUTING.md

+# How to Contribute to r2ogs5
+
+We are happy about all contributions!
+
+
+## Bug reports?
+
+- Please open an issue for the bug [here](https://gitlab.opengeosys.org/ogs/tools/r2ogs6/-/issues), set the *bugfix* label, add a clear problem description, and, if possible add a
+[minimal working example](https://en.wikipedia.org/wiki/Minimal_working_example).
+
+
+## New features?
+
+Open a new [issue](https://gitlab.opengeosys.org/ogs/tools/r2ogs6/-/issues)
+with your idea.
+
+
+## Contribute code?
+
+- Clone the [repo](https://gitlab.opengeosys.org/ogs/tools/r2ogs6.git).
+- Add yourself as author or contributor to DESCRIPTION.
+- Add your code.
+- Push to the repo and submit a merge request.