Document /ko-app/ and entrypoint in README.md

[imjasonh]

Fixes #496

[codecov-commenter]

Codecov Report

Merging #499 (274ec30) into main (de2ce53) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #499   +/-   ##
=======================================
  Coverage   50.15%   50.15%           
=======================================
  Files          41       41           
  Lines        1944     1944           
=======================================
  Hits          975      975           
  Misses        809      809           
  Partials      160      160           

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