Update README to show metrics example

- Also update the actual CFG file from the example

README.rst

@@ -65,7 +65,9 @@ like:
   objectives = ['accuracy']
 
   [Output]
-  # again, these can/should be absolute paths
+  # Also compute the area under the ROC curve as an additional metric
+  metrics = ['roc_auc']
+  # The following can/should be absolute paths
   log = output
   results = output
   predictions = output

examples/titanic/evaluate_tuned.cfg

@@ -18,7 +18,8 @@ objectives = ['accuracy']
 
 [Output]
 # again, these can be absolute paths
+metrics = ['roc_auc']
 log = output
 results = output
 predictions = output
-models = output
+models = output