Add changelog

CHANGELOG.rst

+Changelog
+=========
+
+All notable changes to this project will be documented in this file.
+
+The format is based on `Keep a Changelog`_,
+and this project adheres to `Semantic Versioning`_.
+
+`2.0b0`_ - 2021-11-03
+--------------------
+
+Added
+~~~~~
+
+* Provide ``Poster`` model for time-based documents.
+  * Organise posters in poster groups.
+  * Return current poster of a poster group as PDF file under a specific endpoint.
+* Provide ``LiveDocument`` for periodically updated documents.
+
+
+.. _Keep a Changelog: https://keepachangelog.com/en/1.0.0/
+.. _Semantic Versioning: https://semver.org/spec/v2.0.0.html
+
+.. _2.0b0: https://edugit.org/AlekSIS/Official/AlekSIS/-/tags/2.0b0