Update stacking citation in inst/CITATION

[jgabry]

Closes #283

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.54%. Comparing base (765d409) to head (fd5587b).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #284   +/-   ##
=======================================
  Coverage   92.54%   92.54%           
=======================================
  Files          31       31           
  Lines        3017     3017           
=======================================
  Hits         2792     2792           
  Misses        225      225           

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[jgabry]

The R-devel failure is unrelated to this PR, so will go ahead and merge