Add changes file for Qt 5.12.0

Change-Id: I8ef16c14151f4a5b7f8c86d3bc37cb59adc2a940
Reviewed-by: BogDan Vatra <bogdan@kdab.com>

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.
+
+****************************************************************************
+*                                QtAndroid                                 *
+****************************************************************************
+
+ - QAndroidJniEnvironment::findClass convenient method to find classes in
+   all class loaders.