mention using external libs on Windows

git-svn-id: https://svn.r-project.org/R/trunk@67081 00db46b3-68df-0310-9c12-caf00c1e9a41
more on shell restrictions

git-svn-id: https://svn.r-project.org/R/trunk@67082 00db46b3-68df-0310-9c12-caf00c1e9a41