Document that the object of a plugin must be default-constructible.

Change-Id: Iaaf56ec3bb0d2423c8ab5deb0627dc1357cee830
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>

dist/changes-5.0.0

@@ -649,6 +649,9 @@ Qt for Windows CE
 *                          Plugins                                         *
 ****************************************************************************
 - The text codecs that were previously plugins are now built into QtCore.
+- Code using Q_EXPORT_PLUGIN macros will no longer compile. Use
+  Q_PLUGIN_METADATA instead. Note that this requires that the class
+  be default-constructible.
 
 ****************************************************************************
 *                   Important Behavior Changes                             *

src/corelib/plugin/qplugin.qdoc

@@ -77,6 +77,8 @@
 
     See the \l{tools/plugandpaint}{Plug & Paint} example for details.
 
+    Note that the class this macro appears on must be default-constructible
+
     \sa Q_DECLARE_INTERFACE(), {How to Create Qt Plugins}
 */