Document ko publish in README

[imjasonh]

ko build introduced in #456 is not yet released, which leads to confusion.

Before documenting ko build in the README, we should cut a release with ko build, and clarify that ko publish is an alias for compatibility with previous releases.

Fixes #468

[codecov-commenter]

Codecov Report

Merging #469 (a380199) into main (6230daa) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #469   +/-   ##
=======================================
  Coverage   53.13%   53.13%           
=======================================
  Files          36       36           
  Lines        1848     1848           
=======================================
  Hits          982      982           
  Misses        715      715           
  Partials      151      151           

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