Describe what means 'definition of done' for Regressions and Bugs

doc/development/contributing/merge_request_workflow.md

@@ -220,6 +220,8 @@ requirements.
 1. Working and clean code that is commented where needed.
 1. [Unit, integration, and system tests](../testing_guide/index.md) that all pass
    on the CI server.
+1. Regressions and bugs are covered with tests that reduce the risk of the issue happening
+   again.
 1. Performance/scalability implications have been considered, addressed, and tested.
 1. [Documented](../documentation/index.md) in the `/doc` directory.
 1. [Changelog entry added](../changelog.md), if necessary.