Add description to Sprint 23

It is strange to have a blog post listing things without any kind of
introduction. The description in the blog post list also looks strange
as it takes the first title as description.

_posts/development/2017-09-04-sprint-report-8.md

@@ -3,6 +3,9 @@ layout: post
 title: Highlights of the OBS frontend development – Sprint 23
 category: development
 ---
+<p>
+  Here are the results the OBS frontend team has achieved in the last two weeks (2017-08-21 to 2017-09-01).
+</p>
 
 <h2>OBS 2.8.3 Release</h2>