docs: add paularmstrong as a contributor

[allcontributors]

Adds @paularmstrong as a contributor for code, test.

This was requested by kentcdodds in this comment

[codecov-io]

Codecov Report

Merging #276 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #276   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          20     20           
  Lines         273    273           
  Branches       60     60           
=====================================
  Hits          273    273

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