message: add link to decode_bmp
diff: @@ -19,6 +19,7 @@ Note: The PNG encode and decode Ops support RGBA, but the conversions Ops
  presently only support RGB, HSV, and GrayScale. Presently, the alpha channel has
  to be stripped from the image and re-attached using slicing ops.
  +* @{tf.image.decode_bmp}
  * @{tf.image.decode_gif}
  * @{tf.image.decode_jpeg}
  * @{tf.image.encode_jpeg}
  