Add reference to arch diagram in section 1.4

VOTable.tex

@@ -263,6 +263,7 @@ \subsection{VOTable in the VO Architecture}
 \end{figure}
 
 VOTable is a core IVOA standard.
+\Fref{fig:archdiag} shows the role this document plays within the IVOA architecture.
 
 Wherever tabular data is transferred between Virtual Observatory components,
 VOTable provides the preferred serialization format.