Mention about pyserde-timedelta in README.md

[yukinarit]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (47ab8d8) 89.82% compared to head (8117ef7) 89.82%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #461   +/-   ##
=======================================
  Coverage   89.82%   89.82%           
=======================================
  Files          12       12           
  Lines        1937     1937           
  Branches      401      401           
=======================================
  Hits         1740     1740           
  Misses        145      145           
  Partials       52       52           

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