Updated readme with sufficently nasty warning. updated inst-svn_externals to create an svn removal script.

git-svn-id: svn+ssh://civmxsrv2.duhs.duke.edu/Users/civmsvn/svn/repos/workstation_code/trunk@2135 d41079a3-eaa1-4f09-8506-b324c47726bb