Add citation info

CITING.rst

@@ -0,0 +1,21 @@
+Citing
+------
+
+If you use pfsspy in work that results in publication, please cite the
+Journal of Open Source Science paper at https://doi.org/10.21105/joss.02732.
+A ready made bibtex entry is
+
+.. code:: bibtex
+
+  @article{Stansby2020,
+    doi = {10.21105/joss.02732},
+    url = {https://doi.org/10.21105/joss.02732},
+    year = {2020},
+    publisher = {The Open Journal},
+    volume = {5},
+    number = {54},
+    pages = {2732},
+    author = {David Stansby and Anthony Yeates and Samuel T. Badman},
+    title = {pfsspy: A Python package for potential field source surface modelling},
+    journal = {Journal of Open Source Software}
+  }

doc/source/index.rst

@@ -30,17 +30,7 @@ improve code performance. These can be installed with
 
     pip install pfsspy[performance]
 
-Citing
-------
-
-If you use pfsspy in work that results in publication, please cite the archived
-code at *both*
-
-  - https://zenodo.org/record/2566462
-  - https://zenodo.org/record/1472183
-
-Citation details can be found at the lower
-right hand of each web page.
+.. include:: ../../CITING.rst
 
 Contents
 --------

pfsspy/__init__.py

@@ -26,3 +26,19 @@
         distutils.version.LooseVersion("3")):
     raise RuntimeError('pfsspy requires astropy v3 to run ' +
                        f'(found version {astropy.__version__} installed)')
+
+
+__citation__ = __bibtex__ = """
+@article{Stansby2020,
+doi = {10.21105/joss.02732},
+url = {https://doi.org/10.21105/joss.02732},
+year = {2020},
+publisher = {The Open Journal},
+volume = {5},
+number = {54},
+pages = {2732},
+author = {David Stansby and Anthony Yeates and Samuel T. Badman},
+title = {pfsspy: A Python package for potential field source surface modelling},
+journal = {Journal of Open Source Software}
+}
+"""