add behavioural change in closing window to changelog

Author: Chilipp

CHANGELOG.rst

@@ -5,6 +5,9 @@ Changed
 -------
 - The plotmethod tabs have now a more intuitive gridlayout (see
   `#46 <https://github.com/psyplot/psy-view/pull/46>`__)
+- When closing the mainwindow of psy-view now, one closes all open windows (i.e.
+  also the open figures, see
+  `#47 <https://github.com/psyplot/psy-view/pull/47>`__)
 
 Added
 -----