Update README to remove periodic boundaries as a limitation

[paulromano]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.86%. Comparing base (8fa5b9b) to head (ec143d5).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #54   +/-   ##
=======================================
  Coverage   86.86%   86.86%           
=======================================
  Files           3        3           
  Lines         731      731           
  Branches      191      191           
=======================================
  Hits          635      635           
  Misses         52       52           
  Partials       44       44           

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