Added a caveat to ntfs-3g formula to prevent confusion about mounting.

When installed the ntfs-3g package I went through a lot of confusion trying to get OSX to use it to mount ntfs partitions. It turned out that if you want to use it through the normal mount command or through disk utility, you need to symlink the prefixed version of mount_ntfs to /sbin/.

I thought adding a caveat might help people with the installation process. The osxfuse has a similar caveat already.

What do you think?