readme: Introduction
  See https://github.com/k2-fsa/icefall/pull/330
  and https://github.com/k2-fsa/icefall/pull/452
  
  It has random combiner inside.
  
  Note: There is something wrong in the log file, which has been fixed in https://github.com/k2-fsa/icefall/pull/468.
  