Add link to contribution page

Author: davidpiegza

index.html

@@ -78,6 +78,12 @@ <h2>examples</h2>
           <div>
             <a href="examples/sphere_graph/index.html" target="viewer">sphere graph</a>
           </div>
+
+          <br />
+
+          <div>
+            <a href="https://github.com/davidpiegza/Graph-Visualization#contribution">add more examples</a>
+          </div>
         </section>
       </aside>