Add doc for dijkstra
diff --git a/src/dijkstra.rs b/src/dijkstra.rs
index fe9c401..e7b6feb 100644
--- a/src/dijkstra.rs
+++ b/src/dijkstra.rs
@@ -31,6 +31,55 @@
/// cost is calculated.
///
/// Returns a `HashMap` that maps `NodeId` to path cost.
+/// # Example
+/// ```rust
+/// use petgraph::Graph;
+/// use petgraph::algo::dijkstra;
+/// use petgraph::prelude::*;
+/// use std::collections::HashMap;
+///
+/// let mut graph : Graph<(),(),Directed>= Graph::new();
+/// let a = graph.add_node(()); // node with no weight
+/// let b = graph.add_node(());
+/// let c = graph.add_node(());
+/// let d = graph.add_node(());
+/// let e = graph.add_node(());
+/// let f = graph.add_node(());
+/// let g = graph.add_node(());
+/// let h = graph.add_node(());
+/// // z will be in another connected component
+/// let z = graph.add_node(());
+///
+/// graph.extend_with_edges(&[
+/// (a, b),
+/// (b, c),
+/// (c, d),
+/// (d, a),
+/// (e, f),
+/// (b, e),
+/// (f, g),
+/// (g, h),
+/// (h, e)
+/// ]);
+/// // a ----> b ----> e ----> f
+/// // ^ | ^ |
+/// // | v | v
+/// // d <---- c h <---- g
+///
+/// let expected_res: HashMap<NodeIndex, usize> = [
+/// (a, 3),
+/// (b, 0),
+/// (c, 1),
+/// (d, 2),
+/// (e, 1),
+/// (f, 2),
+/// (g, 3),
+/// (h, 4)
+/// ].iter().cloned().collect();
+/// let res = dijkstra(&graph,b,None, |_| 1);
+/// assert_eq!(res, expected_res);
+/// // z is not inside res because there is not path from b to z.
+/// ```
pub fn dijkstra<G, F, K>(graph: G, start: G::NodeId, goal: Option<G::NodeId>,
mut edge_cost: F)
-> HashMap<G::NodeId, K>
