Add docs about Schleuder version 3.5.3

_posts/2020-06-13-schleuder-3-5-3-released.md

+---
+layout: post
+title:  "Schleuder 3.5.3 released"
+date: "2020-06-13"
+---
+
+This release ships a bug fix, which is not relevant for use in production:
+
+* Allow to run specs on IPv6-only machines.
+
+Please see the [user-relevant changes]({{ "schleuder/docs/changes.html" | absolute_url }}) and the [changelog](https://0xacab.org/schleuder/schleuder/blob/master/CHANGELOG.md#353-2020-06-13) for details.