Point out that g_type_init() is required

docs/reference/gio/overview.xml

@@ -182,6 +182,12 @@
       To do so, use the <filename>gio-unix-2.0.pc</filename> file
       instead of <filename>gio-2.0.pc</filename>
     </para>
+
+    <para>
+      Since GIO is based on GObject, you need to call g_type_init()
+      before you can use any GIO functions. If your application uses
+      GTK+, this is already taken care of by gtk_init().
+    </para>
   </chapter>
 
   <chapter>