Add introduction to the model with metadata format

PiperOrigin-RevId: 360988562
Change-Id: Ia237514735eae86733d8bd27c3dd93f1e7c1d04b

tensorflow/lite/g3doc/convert/metadata.md

@@ -16,6 +16,31 @@ All image models published on
 and [TensorFlow Hub](https://tfhub.dev/s?deployment-format=lite) have been
 populated with metadata.
 
+## Model with metadata format
+
+<center><img src="../images/convert/model_with_metadata.png" alt="model_with_metadata" width="70%"></center>
+<center>Figure 1. TFLite model with metadata and associated files.</center>
+
+Model metadata is defined in
+[metadata_schema.fbs](https://github.com/tensorflow/tflite-support/blob/master/tensorflow_lite_support/metadata/metadata_schema.fbs),
+a
+[FlatBuffer](https://google.github.io/flatbuffers/index.html#flatbuffers_overview)
+file. As shown in Figure 1, it is stored in the
+[metadata](https://github.com/tensorflow/tensorflow/blob/bd73701871af75539dd2f6d7fdba5660a8298caf/tensorflow/lite/schema/schema.fbs#L1208)
+field of the
+[TFLite model schema](https://github.com/tensorflow/tensorflow/blob/master/tensorflow/lite/schema/schema.fbs),
+under the name, `"TFLITE_METADATA"`. Some models may come with associated files,
+such as
+[classification label files](https://github.com/tensorflow/examples/blob/dd98bc2b595157c03ac9fa47ac8659bb20aa8bbd/lite/examples/image_classification/android/models/src/main/assets/labels.txt#L1).
+These files are concatenated to the end of the original model file as a ZIP
+using the ZipFile
+["append" mode](https://pymotw.com/2/zipfile/#appending-to-files) (`'a'` mode).
+TFLite Interpreter can consume the new file format in the same way as before.
+See [Pack the associated files](#pack-the-associated-files) for more
+information.
+
+See the instruction below about how to populate, visualize, and read metadata.
+
 ## Setup the metadata tools
 
 Before adding metadata to your model, you will need to a Python programming