added citation guidelines

CITATION.rst

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+Citation Guidelines
+===================
+
+|JOSS| |Birrer&Amara|
+
+If you use lenstronomy for work or research presented in a publication (whether
+directly, or as a dependency of another package) we recommend and encourage
+the following acknowledgment:
+
+  This research made use of lenstronomy, a multi-purpose gravitational lens
+  modeling software package (Birrer et. al., 2021, Birrer & Amara, 2018).
+
+where the citations are to our publication in the `Journal of Open Source
+Software`_ and the `Original Publication`_. We also encourage citations within the main text wherever
+appropriate. DOIs and BibTeX keys are available through the links above.
+
+Please also cite `Birrer et al 2015 <http://adsabs.harvard.edu/abs/2015ApJ...813..102B>`_
+when you make use of the lenstronomy work-flow or the Shapelet source reconstruction and make sure to cite also
+the relevant work that was implemented in lenstronomy, as described in the release paper and the documentation.
+Don't hesitate to reach out to the developers if you have questions!
+
+
+.. _Journal of Open Source Software: https://joss.theoj.org/papers/10.21105/joss.03283
+.. _Original Publication: https://ui.adsabs.harvard.edu/abs/2018PDU....22..189B/abstract
+
+
+.. |JOSS| image:: https://joss.theoj.org/papers/10.21105/joss.03056/status.svg
+    :target: https://doi.org/10.21105/joss.03056
+
+.. |Birrer&Amara| image:: https://img.shields.io/badge/arXiv-1803.09746%20-yellowgreen.svg
+    :target: https://ui.adsabs.harvard.edu/abs/2018PDU....22..189B/abstract