Update Reader documentation


git-svn-id: svn://svn.r-forge.r-project.org/svnroot/tm@1337 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Add high-level Reader description


git-svn-id: svn://svn.r-forge.r-project.org/svnroot/tm@1338 edb9625f-4e0d-4859-8d74-9fd3b1da38cb