Add changes file.

List the most relevant changes.

Change-Id: Ia61bd528e443ede5bef75dd61e68adc3ae637bb1
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>

dist/changes-5.5.2

+Qt 5.5.2 is a bug-fix release. It maintains both forward and backward
+compatibility (source and binary) with Qt 5.5.0.
+
+Qt 5.5 introduces many new features and improvements as well as bugfixes
+over the 5.4.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 5.5 series is binary compatible with the 5.4.x series.
+Applications compiled for 5.4 will continue to run with 5.5.
+
+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.
+
+****************************************************************************
+*                          General                                         *
+****************************************************************************
+
+QtWebEngineCore
+--------
+ - [QTBUG-48309] Fixed old-style QObject connections to
+   QWebEngineDownloadItem::stateChanged.
+ - [QTBUG-47976] Fixed httpUserAgent setting after the view's
+   initialization.
+ - [QTBUG-48206] Parse suggested filename from content-disposition response
+   header field.
+ - Fixed crash on malformed URL (Chromium issue 533361).
+
+****************************************************************************
+*                      Platform Specific Changes                           *
+****************************************************************************
+
+OS X
+----
+ - [QTBUG-48228] Enabled 32 bit build.
+
+Windows
+-------
+ - [QTBUG-48285] Fixed installation of PDB files.
+