Add changelog.

CHANGELOG.rst

+Changelog
+=========
+
+`1.0a1`_
+--------
+
+New features
+~~~~~~~~~~~~
+
+* Support lesson cancellation
+* Devs: Add fully pythonic API for calendar weeks
+
+Bugfixes
+~~~~~~~~
+
+* Redirect to correct date after editing a substitution
+* Correctly display teachers for substituted lessons
+
+Minor changes
+~~~~~~~~~~~~~
+
+* Use bootstrap buttons everywhere
+
+_`1.0a1`: https://edugit.org/Teckids/BiscuIT/BiscuIT-App-Chronos/-/tags/1.0a1