Point contributors to coordinate conventions

CONTRIBUTING.txt

@@ -152,6 +152,10 @@ Stylistic Guidelines
 * When documenting array parameters, use ``image : (M, N) ndarray``
   and then refer to ``M`` and ``N`` in the docstring, if necessary.
 
+* Refer to array dimensions as (plane), row, column, not as x, y, z. See
+  `Coordinate conventions <http://scikit-image.org/docs/dev/user_guide/numpy_images.html#coordinate-conventions>`__
+  in the user guide for more information.
+
 * Functions should support all input image dtypes.  Use utility functions such
   as ``img_as_float`` to help convert to an appropriate type.  The output
   format can be whatever is most efficient.  This allows us to string together