Add changelog for 5.7.0

Change-Id: Iccb857b2d7ba66a89832c24a04657294ff2d166f
Reviewed-by: Jani Heikkinen <jani.heikkinen@qt.io>

dist/changes-5.7.0

+Qt 5.7 introduces many new features and improvements as well as bugfixes
+over the 5.6.x series. For more details, refer to the online documentation
+included in this distribution. The documentation is also available online:
+
+  http://doc.qt.io
+
+The Qt version 5.7 series is binary compatible with the 5.6.x series.
+Applications compiled for 5.6 will continue to run with 5.7.
+
+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.
+
+****************************************************************************
+*                                 General                                  *
+****************************************************************************
+ - This release contains only minor improvements.