added reference for Yen algorithm

rustworkx-core/src/shortest_path/simple_shortest_paths.rs

@@ -9,7 +9,7 @@
 // WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 // License for the specific language governing permissions and limitations
 // under the License.
-// 
+//
 // This Library is for finding out the shortest paths in increasing cost order.
 
 use crate::petgraph::algo::Measure;
@@ -144,6 +144,8 @@ where
 }
 
 /// Implementation of Yen's Algorithm to find k shortest paths.
+/// More on Yen's algorithm - https://people.csail.mit.edu/minilek/yen_kth_shortest.pdf
+
 pub fn get_smallest_k_paths_yen<N, K, T>(
     graph: &mut Graph<N, K, T>,
     start: NodeIndex,