Updating README with training and inference examples

[ramanakumars]

Need to update README with more training and inference examples.

• Add training information
  • List of required and optional keys
  • Documentation on model architecture
  • Example YAML file for training + documentation on downloading test set
  • Creating a Dataset class
• Adding inference information
  • List of required and optional keys
  • Example YAML file for inference
  • Creating an inference Dataset class
  • Adding a save_mask method