message: docs: add citation suggestion to docs/index.rst
diff: @@ -15,6 +15,16 @@ All tools are powered by the same algorithms, they consume the same inputs and p
  Nextclade is a part of `Nextstrain <https://nextstrain.org>`_, an open-source project to harness the scientific and public health potential of pathogen genome data. All source code is available on `GitHub <https://github.com/nextstrain/nextclade>`_.
  +---------------------------------------
  +Referring to Nextclade in a publication
  +---------------------------------------
  +
  +If you use results obtained with Nextclade in a publication, we suggest you cite our preprint:
  +
  +**Nextclade: clade assignment, mutation calling and quality control for viral genomes**
  +/Ivan Aksamentov; Cornelius Roemer; Emma B Hodcroft; Richard A Neher/
  +.. image:: https://zenodo.org/badge/DOI/10.5281/zenodo.5607694.svg
  + :target: https://doi.org/10.5281/zenodo.5607694
  .. note::
  Do you find Nextclade useful? Tell us about your use-case and experience with it.
  