[WIP] Migrate NOAA-GFDL/fre-cli's fre/cmor/README.md to this repository

[Codex]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

---

This section details on the original issue you should resolve

<issue_title>migrate NOAA-GFDL/fre-cli's fre/cmor/README.md to this repository</issue_title>
<issue_description>adjust as necessary the content, of course.

if it clashes too much with a base-repo README, it could be labeled QUICKSTART

</issue_description>

Comments on the Issue (you are @codex[agent] in this section)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@           Coverage Diff           @@
##             main      #65   +/-   ##
=======================================
  Coverage   76.81%   76.81%           
=======================================
  Files           9        9           
  Lines        1199     1199           
=======================================
  Hits          921      921           
  Misses        278      278           

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