Regenerate extras and postgis pots/po after recent update of sources

git-svn-id: http://svn.osgeo.org/postgis/trunk@13072 b70326c6-7e19-0410-871a-916f4a2858ee
Add transifex configuration and a few words about using `tx'

See http://docs.transifex.com/developer/client/set

git-svn-id: http://svn.osgeo.org/postgis/trunk@13073 b70326c6-7e19-0410-871a-916f4a2858ee