Doc: Added a link to the wiki with plugin info.

The wiki summarizes each platform plugin and the
supported features.

Change-Id: Id7707306880d7e0612e73b879fc48201e0a97417
Task-number: QTBUG-30381
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>
Reviewed-by: Yoann Lopes <yoann.lopes@theqtcompany.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>

src/multimedia/doc/src/qtmultimedia-index.qdoc

@@ -130,6 +130,11 @@
      \endlist
 
     \section2 Platform Notes
+
+    The \l{Qt Multimedia Backends} wiki provides a summary of features
+    supported by each platform plugin made available by this module. The
+    following topics provide more platform-specific information.
+
      \list
       \li \l{Qt Multimedia on BlackBerry}{BlackBerry}
       \li \l{Qt Multimedia on Windows}{Windows}