Add block diagram

Author: timbl

Documentation/index.html

@@ -44,6 +44,8 @@ <h1>Introductory material</h1>
 
 </ul>
 
+ <img src="diagrams/rdflib-block-diagram.svg" alt="Separate modules perform different functions to provide local query, web access, and live update." />
+ 
 </div>
 </body>
 </html>