message: Update docs for supported modules.
diff: TensorFlow I/O is a collection of file systems and file formats that are not
  available in TensorFlow's built-in support.
  +At the moment TensorFlow I/O supports 4 data sources:
  +- `tensorflow_io.ignite`: Data source for Apache Ignite and Ignite File System (IGFS).
  +- `tensorflow_io.kafka`: Apache Kafka stream-processing support.
  +- `tensorflow_io.kinesis`: Amazon Kinesis data streams support.
  +- `tensorflow_io.hadoop`: Hadoop SequenceFile format support.
  +
  ## Installation
  The `tensorflow-io` package could be installed with pip directly:
  