Add changes file for Qt 5.13.0

Change-Id: Ib30e60cdf0ee286064a589775a6609f44029f2b8
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>

dist/changes-5.13.0

+Qt 5.13 introduces many new features and improvements as well as bugfixes
+over the 5.12.x series. For more details, refer to the online documentation
+included in this distribution. The documentation is also available online:
+
+https://doc.qt.io/qt-5/index.html
+
+The Qt version 5.13 series is binary compatible with the 5.12.x series.
+Applications compiled for 5.12 will continue to run with 5.13.
+
+Some of the changes listed in this file include issue tracking numbers
+corresponding to tasks in the Qt Bug Tracker:
+
+https://bugreports.qt.io/
+
+Each of these identifiers can be entered in the bug tracker to obtain more
+information about a particular change.
+
+ - This release contains only minor code improvements.