Doc: add link to Internationalization with Qt

To the Qt Linguist Manual.

Change-Id: I55b3ccb8fe04b6997c3ce953de6544a193b3548e
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>

src/linguist/linguist/doc/src/linguist-manual.qdoc

@@ -56,6 +56,9 @@
     the context in which phrases appear. Developers can use tutorials to learn
     about their tasks.
 
+    For more information about the supported languages and writing systems, see
+    \l{Internationalization with Qt}.
+
     \section1 Table of Contents
 
     \list