message: Add S3 to the list of implemented file systems in doc
  This fix adds S3 to the list of implemented file systems
  in doc.
diff: @@ -35,6 +35,7 @@ Note that TensorFlow already includes many filesystem implementations, such as:
  * HDFS - the Hadoop File System
  * GCS - Google Cloud Storage filesystem
  +* S3 - Amazon Simple Storage Service filesystem
  * A "memory-mapped-file" filesystem
  The rest of this guide describes how to implement a custom filesystem.
  