l10n: README: call more attention to plural strings

In po/README.md, we point core developers to gettext's "Preparing
Strings" documentation for advice on marking strings for translation.
However, this doc doesn't really discuss the issues around plural form
translation, which can make it seem that nothing special needs to be
done in this case.

Add a specific callout here about marking plural-form strings so that
the advice later on in the README is not overlooked.

Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

po/README.md

@@ -221,6 +221,10 @@ General advice:
 - Adjust the strings so that they're easy to translate. Most of the
   advice in `info '(gettext)Preparing Strings'` applies here.
 
+- Strings referencing numbers of items may need to be split into singular and
+  plural forms; see the Q\_() wrapper in the C sub-section below for an
+  example.
+
 - If something is unclear or ambiguous you can use a "TRANSLATORS"
   comment to tell the translators what to make of it. These will be
   extracted by xgettext(1) and put in the "po/\*.po" files, e.g. from