Update Readme so that .test will work. (#659)

When one follows the Readme, the example will fail once we call `trainer.test()` because the methods are not overridden.

Fixes https://github.com/williamFalcon/pytorch-lightning/issues/428

README.md

@@ -128,6 +128,18 @@ To use lightning do 2 things:
             avg_loss = torch.stack([x['val_loss'] for x in outputs]).mean()
             tensorboard_logs = {'val_loss': avg_loss}
             return {'avg_val_loss': avg_loss, 'log': tensorboard_logs}
+            
+        def test_step(self, batch, batch_idx):
+            # OPTIONAL
+            x, y = batch
+            y_hat = self.forward(x)
+            return {'test_loss': F.cross_entropy(y_hat, y)}
+    
+        def test_end(self, outputs):
+            # OPTIONAL
+            avg_loss = torch.stack([x['test_loss'] for x in outputs]).mean()
+            tensorboard_logs = {'test_loss': avg_loss}
+            return {'avg_test_loss': avg_loss, 'log': tensorboard_logs}
     
         def configure_optimizers(self):
             # REQUIRED