include license change in changelog

Author: Chilipp

CHANGELOG.rst

@@ -8,6 +8,8 @@ CircleCI.
 
 Changed
 -------
+- psyplot is now officially licensed under LGPL-3.0-only,
+  see `#29 <https://github.com/psyplot/psyplot/pull/29>`__
 - Documentation is now hosted with Github Pages at https://psyplot.github.io/psyplot-gui.
   Redirects from the old documentation at https://psyplot-gui.readthedocs.io have been
   configured.