Add ndcube ApJ paper to docs.

docs/acknowledging.rst

@@ -7,6 +7,7 @@ Acknowledging ``ndcube``
 If you use ``ndcube``, we kindly ask you to cite the following references in you publications:
 
 * Ryan, Mumford et al., ndcube: Manipulating N-dimensional Astronomical Data in Python, Journal of Open Source Software, 2023a, DOI: https://doi.org/10.21105/joss.05296
+* Ryan, Mumford et al., A Unified Framework for Manipulating N-dimensional Astronomical Data and Coordinate Transformations in Python: The NDCube 2 & Astropy APE-14 WCS APIs, Astrophysical Journal, 2023b, DOI: https://doi.org/10.3847/1538-4357/ace0bd
 
 Citing these references helps to spread awareness of ``ndcube``'s capabilities and philosophy and
 gives some credit to those who have volunteered their time in making ``ndcube`` possible.