NEWS: Add release dates of previous 3.x releases

Author: matthijskooijman

NEWS.md

@@ -1,16 +1,17 @@
-## Changes in 3.0.2
+## Changes in 3.0.2 (2020-05-19)
 
 * Switch from deprecated xml2po to itstool for translating help files
   (issue 583).
 * Fix off-by-one-day error in CSV exports (issue 576).
 * Support Python3.5 again, this was >= 3.6 (issue 582).
 
-## Changes in 3.0.1
+
+## Changes in 3.0.1 (2020-03-01)
 
 * Fixed a rare crash in hamster-window-server (issue 571).
 
 
-## Changes in 3.0
+## Changes in 3.0 (2020-02-24)
 
 * Fixed dialogs placement (PR 549):
   - dialogs appear above their parent (the overview, if opened).