message: Documentation for the pnm module
diff: +//! Decoding of netpbm image formats (pbm, pgm, ppm and pam).
  +//!
  +//! The formats pbm, pgm and ppm are fully supported. The pam decoder recognizes the tuple types
  +//! `BLACKANDWHITE`, `GRAYSCALE` and `RGB` and explicitely recognizes but rejects their `_ALPHA`
  +//! variants for now as alpha color types are unsupported.
  +
  pub use self::decoder::{PNMDecoder, PNMSubtype};
  mod decoder;
  