Add changes file for 5.7.1

Change-Id: I78ad0038581fea638e47387957169ba7dcf6b636
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

dist/changes-5.7.1

+Qt 5.7.1 is a bug-fix release. It maintains both forward and backward
+compatibility (source and binary) with Qt 5.7.0.
+
+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 5.7 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.
+
+****************************************************************************
+*                          Library                                         *
+****************************************************************************
+
+ - This release contains only minor code improvements.