Add Nix devShell installation guidance

[acevif]

Summary

• add README guidance for installing SwiftFormat in a Nix devShell via nixpkgs
• document using acevif/swiftformat-nix to pin a specific SwiftFormat version

Closes #2473

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.30%. Comparing base (c8e50ff) to head (1307bd4).
⚠️ Report is 17 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2474      +/-   ##
===========================================
- Coverage    95.31%   95.30%   -0.02%     
===========================================
  Files          165      165              
  Lines        25128    25128              
===========================================
- Hits         23950    23947       -3     
- Misses        1178     1181       +3     

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