Fix release procedure to avoid modifying tag

git-svn-id: http://svn.osgeo.org/postgis/branches/2.0@12548 b70326c6-7e19-0410-871a-916f4a2858ee
Updated ChangeLog


git-svn-id: http://svn.osgeo.org/postgis/branches/2.0@12549 b70326c6-7e19-0410-871a-916f4a2858ee
Mention ChangeLog.svn makefile rule

git-svn-id: http://svn.osgeo.org/postgis/branches/2.0@12550 b70326c6-7e19-0410-871a-916f4a2858ee
Add section about POSTGIS_GDAL_* environment variables

git-svn-id: http://svn.osgeo.org/postgis/branches/2.0@12551 b70326c6-7e19-0410-871a-916f4a2858ee
Prepare for release 2.0.6

git-svn-id: http://svn.osgeo.org/postgis/branches/2.0@12552 b70326c6-7e19-0410-871a-916f4a2858ee