dev-docs: added a reference regarding issue closing

dev-docs/index.md

@@ -32,6 +32,7 @@ This reference will help you to get started with Krusader development.
 * [KDE applications release announcements mailing-list](https://mail.kde.org/mailman/listinfo/kde-announce-apps) — [we announce](release-howto.md#send-a-letter-to-mailing-lists) Krusader releases here
 * IRC-channel: chat.freenode.net/kde-devel — for quick questions regarding anything related to KDE development
 * [Git hooks for KDE Bugzilla](https://community.kde.org/Infrastructure/Git/Hooks#Keywords) — use these keywords in your commits
+* [Closing issues through commit messages](https://docs.gitlab.com/ee/user/project/issues/managing_issues.html#closing-issues) — add "Resolved #xxx" in a commit message to close a GitLab issue automatically
 
 ### Krusader dependencies