Document how to add custom overloads

[adrhill]

Closes #163.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.28%. Comparing base (5c4400f) to head (fee794f).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #177   +/-   ##
=======================================
  Coverage   91.28%   91.28%           
=======================================
  Files          37       37           
  Lines        1779     1779           
=======================================
  Hits         1624     1624           
  Misses        155      155           

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