message: Clarify use of ImageReader in top-level doc
  I found that ImageReader::new(Cursor::new(...)) fails to determine the type of an image unless I also call `with_guessed_format()`.  This is clearer in the ImageReader docs, but not in the top-level examples.
diff: //! # let bytes = vec![0u8];
  //!
  //! let img = ImageReader::open("myimage.png")?.decode()?;
  -//! let img2 = ImageReader::new(Cursor::new(bytes)).decode()?;
  +//! let img2 = ImageReader::new(Cursor::new(bytes)).with_guessed_format().decode()?;
  //! # Ok(())
  //! # }
  //! ```
  