Add changes-5.12.0

Change-Id: Iba06924f84455ba928b83e2cc5c851e302b10a0e
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>

dist/changes-5.12.0

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+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:
+
+http://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.
+
+****************************************************************************
+*                               Qt 5.12.0 Changes                          *
+****************************************************************************
+
+QtMQTT
+-----------
+
+ - Added support for MQTT protocol level 5.
+ - [QTBUG-67176] Fixed property overwrite during established connection.
+ - [QTBUG-68614] Fixed management of manually specified secure connection.
+ - [QTBUG-69642] Fixed buffer reading for string data.