Merge branch 'pull-138'

* pull-138:
  DOC: nditer: Add details about why 'readonly' is the operand default
  DOC: nditer: Tweaks to the tutorial based on feedback from Chris
  DOC: nditer: Add links to the nditer introductory doc to make it more discoverable
  DOC: nditer: Finish up the nditer walkthrough with a Cython example
  DOC: nditer: Add tutorial-style material covering more than one operand
  DOC: nditer: Write tutorial-style introduction covering single-array iteration