Clean up contribution analytics doc

[ci skip]

doc/analytics/contribution_analytics.md

 # Contribution Analytics (EE only)
 
-In order to make it easy to see how and where are active in your projects,
-we’re building contribution analytics for GitLab Enterprise Edition.
+_**Note:** This feature was introduced in GitLab EE 8.3._
 
-In GitLab 8.3, you can now quickly get an overview of activity of a group by going to **Group -> Contribution Analytics** at `/groups/groupname/analytics`.
+With contribution analytics you can have an overview for the activity of
+issues, merge requests and push events of your organization and its members.
 
-![See Contribution Analytics in GitLab EE 8.3 and up](group_stats.jpg)
+The analytics page is located at **Group > Contribution Analytics**
+under the URL `/groups/<groupname>/analytics`.
+
+The time period on which the analytics depend on, is spanned in three sections:
+last week, last month and last three months. You can choose which one to
+display by using the dropdown calendar menu in the upper right corner.
+
+![Contribution analytics choose period](img/group_stats_cal.png)
+
+---
+
+There are three main bar graphs that are deducted from the number of
+contributions per group member. These contributions include push events, merge
+requests and closed issues.
+
+You can see below an example of the push events of the
+[gitlab-org][https://gitlab.com/groups/gitlab-org] group.
+
+![Contribution analytics bar graphs](img/group_stats_graph.png)
+
+---
+
+Apart from the bar graphs you can also see the contributions per group member
+which are depicted in a table that can be sorted by:
+
+* Member name
+* Number of pushed events
+* Number of opened issues
+* Number of closed issues
+* Number of opened MRs
+* Number of accepted MRs
+* Number of total contributions
+
+![Contribution analytics contributions table](img/group_stats_table.png)