Add changes file for 5.8.0

Change-Id: Ia2b476603fbe00cd67f98c2a2398432ddf92da36
Reviewed-by: Pasi Keränen <pasi.keranen@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.
+
+****************************************************************************
+*                          Library                                         *
+****************************************************************************
+
+ - [QTBUG-54466] Fix crash when QOpenGLContext::currentContext() is NULL.
+ - [QTBUG-54876] Aligned pixel-row of the image passed to glTexImage2D to
+                 UNPACK_ALIGN.
+ - [QTBUG-56673] Improved software opengl renderer detection.
+ - Fixed quadratic loop in CanvasTextureImageFactory.
+
+****************************************************************************
+*                          Examples                                        *
+****************************************************************************
+
+ - [QTBUG-50393] Fix cellphone example flicking