Add todos

src/main.rs

@@ -20,6 +20,8 @@ struct Graph {
     incidence_vertices: Vec<Vertex>,
 }
 
+// TODO: Add iterator of vertices.
+// TODO: Add iterator of edges.
 // TODO: Add iterator of vertex neighbors, see 'fn add_adjacent()'.
 impl Graph {
     fn vertex_count(&self) -> usize {