Added link to Graph::Undirected

README.md

@@ -4,7 +4,7 @@ Some useful perl modules I have written:
 
     <li>Algorithms::Cube: an object-oriented library for third order tensors (requires Algorithms::Matrix)</li>
 
-    <li>Algorithms::Graph: extends Graph::Undirected (requires Graph::Undirected)</li>
+    <li>Algorithms::Graph: extends Graph::Undirected (requires <a href="http://search.cpan.org/~jhi/Graph/lib/Graph/Undirected.pm">Graph::Undirected</a>)</li>
 
     <li>Algorithms::FCM: fuzzy c-means clustering (requires <a href="https://github.com/ingydotnet/inline-c-pm">Inline::C</a>)</li>