Add changes file for 5.9.0

Change-Id: I9ed5494a8b6b137e4b48f8a788ff89bd3c7a09ac
Reviewed-by: Liang Qi <liang.qi@qt.io>

dist/changes-5.9.0

+Qt 5.9 introduces many new features and improvements as well as bugfixes
+over the 5.8.x series. 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.
+
+TIFF handler
+--------------
+
+ - Add support for reading color indexed and grayscale images stored
+   in tiled format.
+
+WEBP
+----
+
+ - Update the bundled libwebp to version 0.6.0.