Core translation should be documented on po/README

[Bagas Sanjaya <bagasdotme@gmail.com>]

Hello,

I had learned the following when I submitted Indonesian translation [1].

The documentation at po/README missed mentioning about "core translation",
which IMO is minimum translated messages needed to complete the translation.
So the l10n coordinator had to instructed me to complete it.

To translate core messages, I ran `po-helper.sh check id.po` first and
do the translation work on the generated po/core-id.po. Next, I merged
po/core-id.po to id.po by `msgcat`, however he didn't tell me the order of
PO files to be passed to `msgcat`, so AFAIK I `ran msgcat core-id.po id.po`
and fixed "fuzzy" errors (such as misplaced translated messages).

I think these above should be mentioned in po/README.

NOTE: po-helper.sh can be found on po-helper branch of l10n coordinator
repo. [2]

[1]: https://github.com/git-l10n/git-po/pull/500
[2]: https://github.com/git-l10n/git-po/blob/po-helper/utils/po-helper.sh

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