Add changes file for Qt 5.12.0

Change-Id: I016e1d4158385e185248802699f9e04721e9e10a
Reviewed-by: Liang Qi <liang.qi@qt.io>

dist/changes-5.12.0

+Qt 5.12 introduces many new features and improvements as well as bugfixes
+over the 5.11.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 5.12 series is binary compatible with the 5.11.x series.
+Applications compiled for 5.11 will continue to run with 5.12.
+
+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                                   *
+****************************************************************************
+
+ - A number of security-related upstream patches has been applied to the
+   bundled libtiff
+ - Added reading support for 16 bits per channel (rgb48/rgba64) files
+ - [QTBUG-70820] Larger strip size when writing, for improved compression
+
+****************************************************************************
+*                                   WebP                                   *
+****************************************************************************
+
+ - [QTBUG-48628] Retain non-alpha format on reading and writing.
+ - Use default compression setting for lossless (quality==100) writing
+
+****************************************************************************
+*                             Third-Party Code                             *
+****************************************************************************
+
+ - The unmaintained bundled libmng has been removed. Building the mng
+   handler will require libmng to be present as a system or external
+   library.
+ - The unmaintained bundled libjasper has been removed. Building the jp2
+   handler will require libjasper to be present as a system or external
+   library.