Add Tutorial: Finetuning an existing SSD model

Hello! First of all, great work on this awesome lib!

I'm training my own SSDs for object detection, and I noticed there are no tutorials in the docs for fine-tuning a model in the zoo with a different dataset. I think it would be great for newcomers to have a starting point for quick training and results.

There is code for a ToySSD in the Object Detection sector of The Straight Dope, but you can't simply substitute the net in the Notebook for a net from the model_zoo.