add new doc - burndown charts + use cases

- create new doc for bd charts
- add overview and use cases
- add the content from milestones/index.md
- copyedit bd charts existing text

doc/user/project/milestones/burndown_charts.md

+# Burndown Charts
+
+>**Notes:**
+- [Introduced][ee-1540] in [GitLab Enterprise Edition Starter 9.1][ee-9.1].
+- Closed or reopened issues prior to GitLab 9.1 won't have a `closed_at`
+  value, so the burndown chart considers them as closed on the milestone
+  `start_date`. In that case, a warning will be displayed.
+
+## Overview
+
+Burndown Charts are visual representations of the progress of completing a milestone.
+
+![burndown chart](img/burndown_chart.png)
+
+At a glance, you see the current state for the completion a given milestone.
+Without them, you would have to organize the data from the milestone and plot it
+yourself to have the same sense of progress.
+
+GitLab Enterprise Edition Starter plots it for you and presents it in a clear and beautiful chart.
+
+## Use cases
+
+Burndown Charts, in gerenal, are used for tracking and analyzing the completion of
+a milestone. Therefore, their use cases are tied to the
+[use you are giving to your milestone](index.md#use-cases).
+
+To exemplify, suppose you lead a team of developers in a large company,
+and you follow this workflow:
+
+- Your company set the goal for the quarter to deliver 10 new features for your app
+in the upcoming major release
+- You create a milestone, and remind your team to assign that milestone to every new issue
+and merge request part of the launch of your app
+- Every week, you open the milestone, visualize the progress, identify the gaps,
+and help your team to get their work done
+- Every month, you check in with your supervisor, and shows the progress of that milestone
+from the Burndown Chart
+- By the end of the quarter, your team successfully delivered 100% of that milestone, as
+it was taken care of closely throughout the whole quarter
+
+## How it works
+
+A Burndown Chart is available for every project milestone that has a set **start
+date** and a set **due date**.
+
+Find your project's **Burndown Chart** under **Project** > **Issues** > **Milestones**,
+and select a milestone from your current ones.
+
+The chart indicates the project's progress throughout that milestone (for issues assigned to it).
+
+In particular, it shows how many issues were or are still open for a given day in the
+milestone's corresponding period.
+
+Since it only tracks when an issue was last closed (and not its full history), the chart
+assumes that issue was open on days prior to that date. Reopened issues are
+considered as open on one day after they were closed.
+
+Note that with this design, if you create a new issue in the middle of the milestone period 
+(and assign the milestone to the issue), the Burndown Chart will appear as if the 
+issue was already open at the beginning of the milestone. A workaround is to simply 
+close the issue (so that a closed timestamp is stored in the system), and reopen 
+it to ge the desired effect, with a rise in the chart appearing on the day after.
+This is what appears in the example below.
+
+The Burndown Chart can also be toggled to display the cumulative open issue
+weight for a given day. When using this feature, make sure issue weights have
+been properly assigned, since an open issue with no weight adds zero to the
+cumulative value.
+
+[ee-1540]: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1540
+[ee-9.1]: https://about.gitlab.com/2017/04/22/gitlab-9-1-released/#burndown-charts-ees-eep