Add JOSS DOI to README.

README.rst

@@ -19,12 +19,20 @@ ndcube
 .. |Powered by SunPy| image:: http://img.shields.io/badge/powered%20by-SunPy-orange.svg?style=flat
    :target: http://www.sunpy.org
    :alt: SunPy
+.. image:: https://joss.theoj.org/papers/10.21105/joss.05296/status.svg
+   :target: https://doi.org/10.21105/joss.05296
+   :alt: JOSS
 
 ndcube is an open-source SunPy affiliated package for manipulating, inspecting and visualizing multi-dimensional contiguous and non-contiguous coordinate-aware data arrays.
 
 It combines data, uncertainties, units, metadata, masking, and coordinate transformations into classes with unified slicing and generic coordinate transformations and plotting/animation capabilities.
 It is designed to handle data of any number of dimensions and axis types (e.g. spatial, temporal, spectral, etc.) whose relationship between the array elements and the real world can be described by World Coordinate System (WCS) translations.
 
+Citing ndcube
+=============
+If you use ndcube is your research, we kindly ask you to cite the following papers:
+* 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
+
 Installation
 ============