Add changes file for Qt 5.15.1

+ 1a790ba615 Fix UB in webp decode and memory leak in encoder
+ b179695c8e Bump version
+ bd73521070 fix build on Arch Linux

Change-Id: I0bbdadb94d5fc0fc784d121a42cfeb22dc964c80
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>
(cherry picked from commit 2cacdd5d5c)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

dist/changes-5.15.1

@@ -0,0 +1,20 @@
+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.
+
+ - This release contains only minor code improvements.