That's where they belong. As XDG was more or less broken in previous
xmonad releases, we can assume few people use it so now's the best time
to move files around.
For users of `~/.xmonad`, this only causes intermediate outputs (.o,
.hi) to go elsewhere.
Fixes: https://github.com/xmonad/xmonad/issues/178
Users who are installing stack on systems like Debian stable or Ubuntu
may get really old version of stack—much older than we currently
support. Thankfully, stack can upgrade itself rather easily with one
single command, so mention that.
Fixes: https://github.com/xmonad/xmonad/issues/303