Add ref (#9064)

* Add ref

* Fix pep8

sklearn/datasets/base.py

@@ -499,6 +499,9 @@ def load_digits(n_class=10, return_X_y=False):
 
         .. versionadded:: 0.18
 
+    This is a copy of the test set of the UCI ML hand-written digits datasets
+    http://archive.ics.uci.edu/ml/datasets/Optical+Recognition+of+Handwritten+Digits
+
     Examples
     --------
     To load the data and visualize the images::