message: Add crosslink to github docs.
diff: //! This crate provides native rust implementations of
  //! image encoders and decoders and basic image manipulation
  //! functions.
  +//!
  +//! Additional documentation can currently be found in the
  +//! [README.md file which is most easily viewed on github](https://github.com/image-rs/image/blob/master/README.md).
  #![warn(missing_docs)]
  #![warn(unused_qualifications)]
  