message: Add link to reference implementation
diff: # rage: Rust implementation of age
  age is meant to be a simple, secure and modern encryption tool with small
  -explicit keys, no config options, and UNIX-style composability. The spec is at
  -https://age-tool.com.
  +explicit keys, no config options, and UNIX-style composability.
  +
  +- The spec is at https://age-tool.com.
  +- The reference implementation is https://github.com/FiloSottile/age.
  This implementation is in progress, and things will keep moving around in the
  near future, so it's not yet a good time to contribute, and it's definitely not
  