add JOSS paper badge in README

[valeriupredoi]

Description

@bnlawrence @davidhassell @kmuehlbauer @bmaranville @jjhelmus @woutdenolf @zequihg50 congrats, folks, we just got published 🥳 🍻

Before you get started

• ☝ Create an issue to discuss what you are going to do

Checklist

• This pull request has a descriptive title and labels
• This pull request has a minimal description (most was discussed in the issue, but a two-liner description is still desirable)
• Unit tests have been added (if codecov test fails)
• Any changed dependencies have been added or removed correctly (if need be)
• If you are working on the documentation, please ensure the current build passes
• All tests pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.62%. Comparing base (a201161) to head (1dc71c5).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #196   +/-   ##
=======================================
  Coverage   77.62%   77.62%           
=======================================
  Files          15       15           
  Lines        3124     3124           
  Branches      497      497           
=======================================
  Hits         2425     2425           
  Misses        573      573           
  Partials      126      126           

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