Merge pull request #1573 from jni/contributing-array-coords

DOC: Add array coords and Cython-Python wrapping to new contributors guide

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
+  :ref:`Coordinate conventions <numpy-images-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
@@ -162,6 +166,10 @@ Stylistic Guidelines
 * Use ``Py_ssize_t`` as data type for all indexing, shape and size variables
   in C/C++ and Cython code.
 
+* Wrap Cython code in a pure Python function, which defines the API. This
+  improves compatibility with code introspection tools, which are often not
+  aware of Cython code.
+
 Test coverage
 -------------
 

doc/source/user_guide/numpy_images.txt

@@ -130,6 +130,8 @@ the grayscale image above:
     >>> plt.imshow(cat)
 
 
+.. _numpy-images-coordinate-conventions:
+
 Coordinate conventions
 ----------------------