Update README file.

git-svn-id: https://projects.coin-or.org/svn/CppAD/trunk@3528 0e562018-b8fb-0310-a933-ecf073c7c197
Merge https://github.com/bradbell/cppad

git-svn-id: https://projects.coin-or.org/svn/CppAD/trunk@3530 0e562018-b8fb-0310-a933-ecf073c7c197
Merge https://github.com/bradbell/cppad

git-svn-id: https://projects.coin-or.org/svn/CppAD/trunk@3531 0e562018-b8fb-0310-a933-ecf073c7c197
Merge https://github.com/bradbell/cppad

git-svn-id: https://projects.coin-or.org/svn/CppAD/trunk@3533 0e562018-b8fb-0310-a933-ecf073c7c197
clean_tar.sh: clean directory and then create a tarball copy.