update install instructions to reflect code is now part of repo

git-svn-id: http://svn.osgeo.org/postgis/trunk@12795 b70326c6-7e19-0410-871a-916f4a2858ee
update install instructions to include simple example for testing installation

git-svn-id: http://svn.osgeo.org/postgis/trunk@12796 b70326c6-7e19-0410-871a-916f4a2858ee