[pgpool-committers: 8152] pgpool: Add release notes for Pgpool-II 3.7.21

[Masaya Kawamoto]

Add release notes for Pgpool-II 3.7.21

Branch
------
V3_7_STABLE

Details
-------
https://git.postgresql.org/gitweb?p=pgpool2.git;a=commitdiff;h=397b6197b972511b9c714b4505c48128853475ee

Modified Files
--------------
doc.ja/src/sgml/release-3.6.sgml | 166 +++++++++++++++++++++++++++++++++++
doc.ja/src/sgml/release-3.7.sgml | 177 +++++++++++++++++++++++++++++++++++++
doc/src/sgml/release-3.6.sgml    | 175 ++++++++++++++++++++++++++++++++++++
doc/src/sgml/release-3.7.sgml    | 185 +++++++++++++++++++++++++++++++++++++++
4 files changed, 703 insertions(+)