Add changes file for Qt 5.15.1

Pick-to: dev 5.15
Change-Id: Ia632da30fcddee81d73e7884696338f116931bce
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

dist/changes-5.15.1

+Qt 5.15.1 is a bug-fix release. It maintains both forward and backward
+compatibility (source and binary) with Qt 5.15.0.
+
+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 5.15 series is binary compatible with the 5.14.x series.
+Applications compiled for 5.14 will continue to run with 5.15.
+
+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.
+
+ - Fixed error checking when attempting to connect nonexistent states
+ - Fixed flaky autotests with a delayed event in scion.
+