Add PR verification documentation

.github/workflows/README.md

@@ -4,6 +4,11 @@
 
 ![gh_actions.drawio.png](gh_actions.drawio.png)
 
+## PR verification
+
+To ensure a stable main branch we verify PRs once they are created/updated and again when they are accepted and enter the merge queue. Therefore each verification workflow needs to have both ``pull_request`` and ``merge_group`` set in the trigger conditions.
+Additionally all jobs for the merge queue have to be explicitly added in the [otterdog configuration](https://github.com/eclipse-openbsw/.eclipsefdn/blob/main/otterdog/eclipse-openbsw.jsonnet).
+
 ## General guidelines
 
 1. Don't waste compute time.