message: Polish model maker doc: add more links and fallback solutions to TF.
diff: @@ -13,10 +13,20 @@ links below for guides on how to train the model.
  Supported Tasks | Task Utility
  ------------------------------------------------------------------------------------------------------------------------------------------ | ------------
  -Image Classification [guide](https://www.tensorflow.org/lite/tutorials/model_maker_image_classification) | Classify images into predefined categories.
  -Text Classification [guide](https://www.tensorflow.org/lite/tutorials/model_maker_text_classification) | Classify text into predefined categories.
  -BERT Question Answer [guide](https://www.tensorflow.org/lite/tutorials/model_maker_question_answer) | Find the answer in a certain context for a given question with BERT.
  -Recommendation [demo](https://github.com/tensorflow/examples/blob/master/tensorflow_examples/lite/model_maker/demo/recommendation_demo.py) | Recommend items based on the context information for on-device scenario.
  +Image Classification: [guide](https://www.tensorflow.org/lite/tutorials/model_maker_image_classification), [api](https://www.tensorflow.org/lite/api_docs/python/tflite_model_maker/image_classifier) | Classify images into predefined categories.
  +Object Detection: [api](https://www.tensorflow.org/lite/api_docs/python/tflite_model_maker/object_detector) | Detect objects in real time.
  +Text Classification: [guide](https://www.tensorflow.org/lite/tutorials/model_maker_text_classification), [api](https://www.tensorflow.org/lite/api_docs/python/tflite_model_maker/text_classifier) | Classify text into predefined categories.
  +BERT Question Answer: [guide](https://www.tensorflow.org/lite/tutorials/model_maker_question_answer), [api](https://www.tensorflow.org/lite/api_docs/python/tflite_model_maker/question_answer) | Find the answer in a certain context for a given question with BERT.
  +Audio Classification: [demo](https://github.com/tensorflow/examples/blob/master/tensorflow_examples/lite/model_maker/demo/audio_classification_demo.py), [api](https://www.tensorflow.org/lite/api_docs/python/tflite_model_maker/audio_classifier) | Classify audio into predefined categories.
  +Recommendation: [demo](https://github.com/tensorflow/examples/blob/master/tensorflow_examples/lite/model_maker/demo/recommendation_demo.py), [api](https://www.tensorflow.org/lite/api_docs/python/tflite_model_maker/recommendation) | Recommend items based on the context information for on-device scenario.
  +
  +If your tasks are not supported, please first use [TensorFlow](https://www.tensorflow.org/guide)
  +to retrain a TensorFlow model with transfer learning (following guides like
  +[images](https://www.tensorflow.org/tutorials/images/transfer_learning),
  +[text](https://www.tensorflow.org/official_models/fine_tuning_bert),
  +[audio](https://www.tensorflow.org/tutorials/audio/transfer_learning_audio)) or
  +train it from scratch, and then [convert](https://www.tensorflow.org/lite/convert)
  +it to TensorFlow Lite model.
  ## End-to-End Example
  