Add --file CLI option documentation to README.md.

[christhekeele]

Resolves #384.

[codecov]

Codecov Report

Merging #386 (dde2e0b) into main (b719316) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main      #386   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           46        46           
  Lines         1508      1508           
=========================================
  Hits          1508      1508           

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[dbanty]

Awesome, thanks for contributing the fix! Hopefully that helps the next person get started a little bit quicker ☺️