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.
Add instructions for getting homedir path

This should work for all versions of Windows.
Merge pull request #571 from ilkka/patch-1

Add note about .emacs.d Windows path