Update README.md

* How to retrain multilabel:  

```

bash retrain.sh

```

the retrain.sh calls the multilabel version of retrain.py which is quite different from the singlelabel retrain.py.  

It is possible to change hyperparamters in the retrain.sh but not the Inception model as in the singlelabel version.  

Example for retrain.sh with tuned hyperparameters:

```

python retrain.py \

--bottleneck_dir=bottlenecks \

--how_many_training_steps 50000 \

--model_dir=model_dir \

--output_graph=retrained_graph.pb \

--output_labels=retrained_labels.txt \

--summaries_dir=retrain_logs \

--flip_left_right=True \

--random_scale=20 \

--random_brightness=20 \

--random_crop=20 \

--image_dir=/softpro/ss18/caption/cocoapi/multilabel_outer/ 

```

* **Results:**  

[Trained model (default)](/models/multilabel_default/)  

[Trained model (+Hyperparameters)](/models/multilabel_hp/)