add expand_term/2 to README

[ichiban]

No description provided.

[codecov-commenter]

Codecov Report

Merging #127 (91478d2) into main (c9e2310) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #127   +/-   ##
=======================================
  Coverage   85.63%   85.63%           
=======================================
  Files          18       18           
  Lines        5074     5074           
=======================================
  Hits         4345     4345           
  Misses        648      648           
  Partials       81       81           

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