Add changes file for 5.7.1

Change-Id: I562b0d682b27407754ad23bfb578ed839e80779a
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@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 documentation included in this distribution.
+The documentation is also available online:
+
+  https://doc.qt.io/qt-5/
+
+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.
+
+This release contains all fixes included in the Qt 5.6.2 release.
+
+****************************************************************************
+*                           General                                        *
+****************************************************************************
+  - Updated information on installers, Qt account and license keys.
+  - Updated QML coding conventions to mention semicolon usage.
+  - Fixed incorrect information in supported platforms.