need to add --image_dir to generate_tfrecord.py

[orbidder]

This is a great project and it has helped me a lot with my own work. One issue I had is the Usage guide at the head of generate_tfrecord.py suggests --csv_input and --output_path as arguments, but I couldn't get the tool to run without --image_dir too. The error was a "No such file or directory" return for images I knew were in the folder. Adding --image_dir=/images fixed the issue, so this should be amended to the generate_tfrecord.py script so others know to use it too.