Add due dates docs

doc/user/project/issues/due_dates.md

+# Due dates
+
+> [Introduced][ce-3614] in GitLab 8.7.
+
+Due dates can be used in issues to keep track of deadlines and make sure
+features are shipped on time. Due dates require at least [Reporter permissions][permissions]
+to be able to edit them. On the contrary, they can be seen by everybody.
+
+## Setting a due date
+
+When creating or editing an issue, you can see the due date field from where
+a calendar will appear to help you choose the date you want. To remove it,
+select the date text and delete it.
+
+![Create a due date](img/due_dates_create.png)
+
+A quicker way to set a due date is via the issue sidebar. Simply expand the
+sidebar and select **Edit** to pick a due date or remove the existing one.
+Changes are saved immediately.
+
+![Edit a due date via the sidebar](img/due_dates_edit_sidebar.png)
+
+## Making use of due dates
+
+Issues that have a due date can be distinctively seen in the issues index page
+with a calendar icon next to them. Issues where the date is past due will have
+the icon and the date colored red. You can sort issues by those that are
+_Due soon_ or _Due later_ from the dropdown menu in the right.
+
+![Issues with due dates in the issues index page](img/due_dates_issues_index_page.png)
+
+Due dates also appear in your [todos list](../../../workflow/todos.md).
+
+![Issues with due dates in the todos](img/due_dates_todos.png)
+
+[ce-3614]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3614
+[permissions]: ../../permissions.md#project

doc/workflow/README.md

@@ -10,6 +10,7 @@
 - Issues - The GitLab Issue Tracker is an advanced and complete tool for
   tracking the evolution of a new idea or the process of solving a problem.
   - [Confidential issues](../user/project/issues/confidential_issues.md)
+  - [Due date for issues](../user/project/issues/due_dates.md)
 - [Issue Board](../user/project/issue_board.md)
 - [Keyboard shortcuts](shortcuts.md)
 - [File finder](file_finder.md)