message: Add S3 plugin to the list of file system plugin in doc (add_filesys.md)
  This fix adds S3 plugin to the list of file system plugin in
  doc (add_filesys.md).
diff: @@ -81,6 +81,8 @@ filesystem implementations call their existing libraries. Examples include:
  plugin](https://github.com/tensorflow/tensorflow/blob/master/tensorflow/core/platform/hadoop/hadoop_file_system.h)
  * [GCS
  plugin](https://github.com/tensorflow/tensorflow/blob/master/tensorflow/core/platform/cloud/gcs_file_system.h)
  +* [S3
  + plugin](https://github.com/tensorflow/tensorflow/blob/master/tensorflow/core/platform/s3/s3_file_system.h)
  #### The File interfaces
  