Add 5.4.0 changelog for qtdoc

Change-Id: I46b2ac71473aad572da62c624cd1f15e2daf4670
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>

dist/changes-5.4.0

+Qt 5.4 introduces many new features and improvements as well as bugfixes
+over the 5.3.x series. For more details, refer to the online documentation
+included in this distribution. The documentation is also available online:
+
+  http://qt-project.org/doc/qt-5.4
+
+The Qt version 5.4 series is binary compatible with the 5.3.x series.
+Applications compiled for 5.3 will continue to run with 5.4.
+
+Some of the changes listed in this file include issue tracking numbers
+corresponding to tasks in the Qt Bug Tracker:
+
+  http://bugreports.qt-project.org/
+
+Each of these identifiers can be entered in the bug tracker to obtain more
+information about a particular change.
+
+****************************************************************************
+*                           General                                        *
+****************************************************************************
+
+ - Documentation: Fixed several instances of missing or incorrect
+   information.
+ - Updated documentation for supported platforms, new modules.
+ - Updated documentation for Android, iOS support.
+ - Updated deployment documentation on several platforms.
+ - Updated documentation on source breaks.
+ - Updated licensing documentation.
+ - Prepared documentation to be published under the new
+   domain, doc.qt.io.
+ - Added documentation for HiDPI support.
+ - Improved the examples and tutorials overview pages
+ - Updated scalability documentation.
+ - Removed qtdemo example as obsolete.