Add "ChangeLog" rule, update HOWTO_RELEASE with that recipe

git-svn-id: http://svn.osgeo.org/postgis/trunk@12543 b70326c6-7e19-0410-871a-916f4a2858ee
Reorder release steps to avoid touching repository after tag

git-svn-id: http://svn.osgeo.org/postgis/trunk@12546 b70326c6-7e19-0410-871a-916f4a2858ee