Add changes file for 5.9.1

Change-Id: I2d3cbdad72e97e3b6761c4acff80ec769a5c58bd
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>

dist/changes-5.9.1

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+Qt 5.9.1 is a bug-fix release. It maintains both forward and backward
+compatibility (source and binary) with Qt 5.9.0.
+
+For more details, refer to the online documentation included in this
+distribution. The documentation is also available online:
+
+http://doc.qt.io/qt-5/index.html
+
+The Qt version 5.9 series is binary compatible with the 5.8.x series.
+Applications compiled for 5.8 will continue to run with 5.9.
+
+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.
+
+Render
+------
+
+- Render states now honor the enabled property
+
+Animation
+---------
+
+- Fix support for animating a QColor
+- Support for linear interpolation in clip json
+- Fix animation inifinite looping
+
+Quick3D
+-------
+
+- Fixed memory leak in Quick3DNodeInstantiatorPrivate [QTBUG-61293]
+- Fix nested NodeInstantiator use case