add JOSS paper to docs

[orbeckst]

With #71 published, we should add the paper citation

• README: add citation and use badge

  Markdown:
  [![DOI](https://joss.theoj.org/papers/10.21105/joss.06934/status.svg)](https://doi.org/10.21105/joss.06934)
  
  HTML:
  <a style="border-width:0" href="https://doi.org/10.21105/joss.06934">
    <img src="https://joss.theoj.org/papers/10.21105/joss.06934/status.svg" alt="DOI badge" >
  </a>
  
  reStructuredText:
  .. image:: https://joss.theoj.org/papers/10.21105/joss.06934/status.svg
     :target: https://doi.org/10.21105/joss.06934
  

• CITATION.cff (update)
• documentation (add a section on how to cite)