Update docs for supported modules.

Signed-off-by: Yong Tang <[email protected]>

Author: yongtang

README.md

@@ -5,6 +5,12 @@
 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: