Mention license change in 5.7.0 changelog.

Change-Id: Ie313194cb4c73c51f430e06ea3766005c7378c13
Task-number: QTBUG-53913
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. Also, there is a change in the licensing terms.
+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.
+
+****************************************************************************
+*                          Important License Changes                       *
+****************************************************************************
+
+  This module is no longer available under LGPLv2.1. The libraries are
+  now available under the following licenses:
+    * Commercial License
+    * GNU General Public License v2.0 (LICENSE.GPL2) and later
+    * GNU Lesser General Public License v3.0 (LICENSE.LGPL3)
+
+  The tools are now available under the following licenses:
+    * Commercial License
+    * GNU General Public License 3.0 (LICENSE.GPL3) with exceptions
+      described in The Qt Company GPL Exception 1.0 (LICENSE.GPL3-EXCEPT)