doc: Update tools/icu/README.md

- remove TODOs: the one about defaults has been
addressed, and the one about testing is a work
item that doesn't belong in a doc.
- add some background information

Fixes: https://github.com/nodejs/node/issues/7843

PR-URL: https://github.com/nodejs/node/pull/16939


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>