Add note about .emacs.d Windows path

At least on my Windows 7 machine, `user-emacs-directory` prints `~/.emacs.d`, which is not really useful, since you also then have to realize that the tilde refers to the roaming appdata directory. I think this way of documenting would perhaps be more immediately useful. For probably almost all other platforms the tilde path will work just fine.
https://www.gnu.org/software/emacs/manual/html_node/emacs/Windows-HOME.html