po/README: Update directions for l10n contributors

Some Linux distributions (such as Ubuntu) have their own l10n workflows,
and their translations may be different.  Add notes for this case for
l10n translators.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

po/README

@@ -10,10 +10,26 @@ coordinates our localization effort in the l10 coordinator repository:
 
         https://github.com/git-l10n/git-po/
 
+The two character language translation codes are defined by ISO_639-1, as
+stated in the gettext(1) full manual, appendix A.1, Usual Language Codes.
+
+
+Contributing to an existing translation
+---------------------------------------
 As a contributor for a language XX, you should first check TEAMS file in
 this directory to see whether a dedicated repository for your language XX
 exists. Fork the dedicated repository and start to work if it exists.
 
+Sometime, contributors may find that the translations of their Git
+distributions are quite different with the translations of the
+corresponding version from Git official. This is because some Git
+distributions (such as from Ubuntu, etc.) have their own l10n workflow.
+For this case, wrong translations should be reported and fixed through
+their workflows.
+
+
+Creating a new language translation
+-----------------------------------
 If you are the first contributor for the language XX, please fork this
 repository, prepare and/or update the translated message file po/XX.po
 (described later), and ask the l10n coordinator to pull your work.
@@ -23,6 +39,9 @@ coordinate among yourselves and nominate the team leader for your
 language, so that the l10n coordinator only needs to interact with one
 person per language.
 
+
+Translation Process Flow
+------------------------
 The overall data-flow looks like this:
 
     +-------------------+            +------------------+