Add publication to README

[JBGreisman]

This PR adds references to our paper in the README:

• DOI badge with a hyperlink to the paper
• Adds link to the "References" section, and states that the J Appl Cryst paper should be cited for any work that uses rs

[codecov-commenter]

Codecov Report

Merging #107 (72751ff) into main (0c9d898) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #107   +/-   ##
=======================================
  Coverage   98.72%   98.72%           
=======================================
  Files          41       41           
  Lines        1569     1569           
=======================================
  Hits         1549     1549           
  Misses         20       20           

Flag	Coverage Δ
unittests	98.72% <ø> (ø)

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