Replace README.rst with Markdown

[hugovk]

I find Markdown easier to use, especially for README files.

[codecov-commenter]

Codecov Report

Merging #479 (8a7fa8c) into main (1dda667) will not change coverage.
The diff coverage is n/a.

❗ Current head 8a7fa8c differs from pull request most recent head d66342b. Consider uploading reports for the commit d66342b to get more accurate results

@@           Coverage Diff           @@
##             main     #479   +/-   ##
=======================================
  Coverage   88.96%   88.96%           
=======================================
  Files           6        6           
  Lines        1685     1685           
=======================================
  Hits         1499     1499           
  Misses        186      186           

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