Add changes file for 5.8.0

Change-Id: Iad8edc1ce474907a017d2751650a447280e68aaa
Reviewed-by: Antti Kokko <antti.kokko@qt.io>

dist/changes-5.8.0

+Qt 5.8 introduces many new features and improvements as well as bugfixes
+over the 5.7.x series. 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 5.8 series is binary compatible with the 5.7.x series.
+Applications compiled for 5.7 will continue to run with 5.8.
+
+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.
+
+QtImageFormats
+--------------
+
+ - For security reasons, the DDS handler will no longer be built by
+   default. Users who still want this handler can build it from the
+   source project.
+
+TGA
+---
+
+ - Fixed reading of TGA-16 formats.