add required permission to label doc

doc/workflow/labels.md

 # Labels
 
-In GitLab, you can easily tag issues and merge requests. To create a label, go to a project and then to `Issues` and then `Labels`.
+In GitLab, you can easily tag issues and merge requests. If you have permission level `Developer` or higher, you can manage labels. To create, edit or delete a label, go to a project and then to `Issues` and then `Labels`.
 
 Here you can create a new label.