Add milestones documentation to workflow

doc/workflow/README.md

@@ -14,5 +14,6 @@
 - [Protected branches](protected_branches.md)
 - [Web Editor](web_editor.md)
 - [Releases](releases.md)
+- [Milestones](milestones.md)
 - [Merge Requests](merge_requests.md)
 - ["Work In Progress" Merge Requests](wip_merge_requests.md)

doc/workflow/milestones.md

+# Milestones
+
+Milestone allows you to group issues and set due date for it. 
+Milestone is created per project. 
+
+![milestone form](milestones/form.png)
+
+## Groups and milestones
+
+You can create milestone with single form for several projects that belongs to the same group.
+On the group milestones page you will be able to see this milestones grouped together by name.
+
+![group milestone form](milestones/group_form.png)