Add changelog.

CHANGELOG.rst

+Changelog
+=========
+
+`1.0a1`_
+--------
+
+New features
+~~~~~~~~~~~~
+
+* Allow updating subjects, rooms and time periods from new import
+* Allow importing a new version of a timetable
+
+Bugfixes
+~~~~~~~~
+
+* Work around bug in Untis that wrongly splits classes if they contain
+  spaces
+
+Minor changes
+~~~~~~~~~~~~~
+
+* Use bootstrap buttons everywhere
+
+_`1.0a1`: https://edugit.org/Teckids/BiscuIT/BiscuIT-App-Untis/-/tags/1.0a1