More detailed\understandable description for new options

[ropucyka]

Which added in 0.5.0 version:

      --selective
          Skip some DCT coefs for JPEG
      --depth <DEPTH>
          Depth (least bit to use) [default: 0]
      --bits <BITS>
          Number of bits per single image unit (pixel/DCT coef) [default: 1]
      --jpeg-comp <JPEG_COMP>
          JPEG component index
      --max-step <MAX_STEP>
          Overwrite calculated max step

Request for typical or advances users, but who not JPEG expert.