Add docs on milestone percentage stats

Clarify how the milestone percentage is calculated in the docs.
Include example image of milestone.

doc/user/project/milestones/index.md

@@ -44,3 +44,11 @@ special options available when filtering by milestone:
 * **Started** - show issues or merge requests from any milestone with a start
   date less than today. Note that this can return results from several
   milestones in the same project.
+
+## Milestone progress statistics
+
+Milestone statistics can be viewed in the milestone sidebar. The milestone percentage statistic
+is calculated as; closed and merged merge requests plus all closed issues divided by
+total merge requests and issues.
+
+![Milestone statistics](img/progress.png)