Adding Japanese README

I would like to propose to add Japanese translated version of README
in the source tree(for example against backend/storage/buffer/README).
Benefits of our community include:

- Increase possibility to hire more Japanese speaking developers by
helping them to understand source code using those README written in
their local language

- Make Japanese speaking developers life easier. Most of them can read
English README of course. However, reading Japanese one will
increase their productivity thus more contribution to PostgreSQL

- Including Japanese README in the source tree rather than placing in
somewhere else helps syncing English README with Japanese one.

Please note that I am not against adding README translated in other
language. I just don't understand other than Japanese.

Comments?
--
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php
Japanese: http://www.sraoss.co.jp