add link to the dataset (#44)

What data set to use? Week 3 Day 3 ex 5 #36

piscine/week03/day03/ex05/README.md

@@ -3,6 +3,7 @@
 The goal of this exercise is to learn to use a neural network to classify a multiclass data set. The data set used is the Iris data set which allows to classify flower given basic features as flower's measurement. 
 
 Preliminary: 
+    - [Load the dataset from `sklearn`.](https://scikit-learn.org/stable/auto_examples/datasets/plot_iris_dataset.html)
     - Split train test. Keep 20% for the test set. Use `random_state=1`. 
     - Scale the data using Standard Scaler